When I rediscovered my childhood love of Beauty and the Beast, it made intense a disagreement I had already had with a lot of its fans: the idea that the Beast should never have transformed back into his human self (officially called Prince Adam in supplemental material). At first, I disagreed on sheer abstract principle: I was so damn sick and tired of stories of human women being able to fall in love with strange male beings that I didn't want Beauty and the Beast to go any farther than it already had. Afterwards, I realized that to desire the Beast to remain untransformed goes against both what the film was actually building up towards, and what the character himself actually wanted. In other words, it's an arbitrary desire based on personal sentimentality rather than the actual contents of the film.
For some fans, it's just about being unable to empathize with a character that has a different body and voice type than the one they had spent the entire film experiencing. I can understand that--after all, I've been there. Some simply find the prince ugly on his own, which again I can see, though I very much don't share the opinion. Other times, however, dislike of the Beast's transformation is phrased in terms of spiritual enlightenment: the film ought to have taken its endorsement of love transcending appearance to the logical extreme, and have Belle live completely with the persona she had fallen in love with, because she had done it in spite of his monstrous visage.
Fantastical xenophilia is often used as a metaphor about accepting others for their differences, or transcending petty physical concerns for the higher realm. However, when it is always about a human female learning to love a male being of another species/form/attractiveness level, all the progressive implications are erased, and it seems more like a male fantasy, designed to coddle the egos of certain men, by telling them that women are too pure and good to care about looks. This is why I don't endorse Beauty and the Beast on those terms; what it is telling us on those terms is nothing shocking or new.
(This is also my response to anyone who complains that Quasimodo didn't end up with Esmeralda in The Hunchback of Notre Dame)
However, this aspect of xenophilia/transcendence actually isn't the main focus of Beauty and the Beast. While in the prologue, the enchantress warned the prince not to be deceived by appearances, and certainly such messages can be derived from the titular romance, the main message is actually "Stop being a dick and you will get a beautiful girl to fall in love with you" not, "Learn to love yourself as a Beast".
Belle does learn to accept the Beast, but it doesn't really change her character. The true character development is instead in the Beast's nature, and does not involve his learning to accept his appearance as permanent. Instead, he learns not to be an overgrown brat, explicitly so that he can become human again. Although he learned to be a nice person while still a Beast, his monstrous state was originally a symbol of his nasty nature. To become human, therefore, symbolizes his success at overcoming this nature.
Not only is this transformation the main focus of the film, it is something the Beast actively desires. He keeps Belle around because he wants the curse to be broken, and freaks out when she almost touches the Rose, his timetable for when the transformation will become permanent. He also shows active distaste for his form, shredding his own portrait, locking away Belle's father after he thinks Maurice is staring at him, and referring to himself as a "monster". Finally, when he is at last returned to human form, he is overjoyed.
In short, everything in the film points towards the Beast wanting to become human again, and there is no reason given as to why he would not wish it so. In fact, if you listen to the audio commentary for the film, one of the intentions for the story was that the Beast was losing more of his human nature as time went on, to the point where he might have eventually lost his sapience.
Because of all this, it becomes petty to demand a film change everything it was building up to, and a main character change everything they wanted. There are many stories where characters have their goals altered or thwarted, but in the best cases it is built up by the plot and supported by the story's themes. To simply introduce failure in the end without reason or support is bad writing. As Beauty and the Beast exists now, there is nothing that suggests that the Beast would or should remain untransformed in the final reel, and to do so would make the film far too dark and too much of a waste.
Compare it with Shrek. When this movie tries to subvert the Beauty and the Beast story by Fiona turning permanently into an ogre instead of the human princess she expected, it's foreshadowed and endorsed by events in the story. Since Shrek is an allegedly egalitarian subversion of the Disneyfied fairy tales, the audience expects Fiona's, "This is not how a princess is supposed to look" line to be challenged. In addition, her anguish about her ogre form already seems more flimsy than the Beast's equivalent, in light of the fact that Shrek is an ogre, too, and she has already become attracted to him.
Furthermore, Fiona is marrying Farquaad, who is clearly wrong for her, just to break the curse, meaning that working to break the curse is not associated with positive character development as in Beauty and the Beast, but in unnecessary negativity. Learning to accept herself as an ogre coincides with general positive development, i.e. finally rejecting Farquaad and accepting Shrek as her lover, the man she was actually having a good time with. Thus, when Fiona remains an ogre, it is actually what the plot was leading towards. Beauty and the Beast did not create the same expectations.
Many people who protest the Beast's return to human form might instead be considering it from Belle's POV. Belle, like a good chunk of the audience, has fallen in love with the Beast as he is, and so must find it difficult to accept the change. And she briefly does, her face quirking as she looks at the new human form, only accepting him when she sees the same eyes that he has always had.
It might provide more fuel for the anti-prince fires: Belle having brief difficulty accepting his form for a second is transformed into a daydream that he remained a Beast. However, Disney's Beauty and the Beast is unusual in that while Belle is the viewpoint character in the beginning, the Beast is the character who develops the most, and who is considered the actual protagonist. Therefore, any of his desires trump Belle's when it comes to what form he has: because he is the protagonist, the Beast "deserves" to dictate what will happen with his body, and as we've already seen, he wants to become human. Or if you want, simple logic: who decides what their ideal body should be--the owner of the body, or the person who's fallen in love with it? The owner, of course. In addition, if it is about love transcending appearances, then Belle does should not care what he looks like either way, right?
Alternatively, male viewers might look on the Beast as a power fantasy and feel disappointed when it is taken away. But again, this is not what the character or the narrative wants, so the desires of viewers are moot.
Essentially, the Beast's returning to human form is the only way the story could have ended. To have it otherwise would be like Star Wars without the Death Star blowing up. The Beast is the protagonist, he wants to be human, already has a lesson to learn about emotional control, and so there is no reason for him to permanently remain a Beast once he has succeeded in his quest to not be one. Love beyond appearances already exists in Beauty and the Beast (for women, at least), and it must eventually stand aside for the plot development and the fulfillment of a character’s goals.
Not to mention it would seem pretty half-assed to have him stay a Beast, while the rest of the castle's servants transform back into humans and the castle to shed its semi-grotesque nature. If one wanted the Beast to remain a Beast, they would have to be watching a far different film than the one that actually exists.