To Defend Ms Watson's performance for a moment, I think we all forget that Belle is not a princess known for her song. In actuality she is one of 3 princesses who do not actually have an iconic "I Want" song. These include Jasmine, whose song is actually the Duet A Whole New World with Aladdin, as she is not the protagonist. Add to that when Alladin was adapted for the stage I think they went through 2 or 3 different "I want" songs for her before settling on These Palace Walls. The other is Merida who doesn't sing.
Yes Belle has the Belle Reprise, which is 1 minute and 3 seconds of mostly spoken or softly song lyrics except for the following two lines "I want much more than this provincial life/ I want adventure in the great wide somewhere". If you actually listen to the original animated version or even the original Broadway version, its not so much Belle's vocals getting more powerful, as it is the orchestra swelling.
The lead vocal performance of Beauty and the Beast, has never actually been Belle's. In the original movie it was Lumiere and Mrs Potts, and in the Original Broadway adaptation it was Beast. Belle didn't even have a vocal moment outside of Belle in the Broadway adaptation until Toni Braxton joined the production and A Change in Me was added to the score.
Do I think Emma Watson's vocal capabilities are the reason why A Change in Me, was dropped from the live action adaptation? Partially I do, but it also would not work in the live action film, as it would come right after the film stopping Forever More, causing the movie to come to a stop for 2 big ballads.
In all honesty Watson is a fine Belle, and people need to stop confusing Belle with the vocal Powerhouse Princesses of Ariel (Part of Your World, Mulan (Reflection), Pocahontas (Colors of the Wind), Tiana (Almost There), Rapunzel (When Will my Life Begin), Anna (For the First Time in Forever), Elsa (Let it Go), Moana (How Far I'll Go), as Belle does not have a song that those pipes are need for.