It's simply the human nature of valuing what is lost. People who were fans, or were at least somewhat attached to the original cast, are going to now feel more value and attachment to said cast, now that they are missing. Humans fundamentally fixate on the loss of something, rather than the gain of another. We're loss adverse. So that takeaway feels harder for those who were attached to the original cast, versus those who hated or were completely neutral to said cast.
I didn't like it either, honestly. But at the same time, seeing the talent of the new cast, their passion, and the grace and professionalism from the old cast has frankly gotten me to move on. The fact is it's already been done. The voicing has been completed and there's nothing that can be done about it.