It depends on what you think the purpose of keeping creative works outside of the public domain is. Generally, the idea is so that the original creator can make a living off of their art without someone immediately copying their work and undercutting them. The idea of keeping a character true to the original interpretation is not usually considered in this discussion.

Personally, I believe that IP should enter the public domain way sooner than it actually does. I'm generally in favor the original definition of 14 years, with a 14 year extension before the work enters public domain. That gives someone 28 years to make a living off of a character before the ideas become free game for others to use and adapt in any way they see fit.

Having Spongebob as IP keeps him on rails for who he is as a character. Change that, Spongebob as a character is changed by the public that could make the original unrecognizable

I fundamentally disagree with this premise. The vast majority of characters that are in the public domain are not significantly different from their source work, outside of a handful of modern exceptions. Dracula is still mostly Dracula, even in the modern day. Same for Sherlock Holmes, or anyone in a Shakespeare play. The idea of completely twisting a character once they enter the public domain happens, like with Blood and Honey, or that Popeye horror movie coming out, but I think you'd struggle to find anyone that only knows Winnie the Pooh or Popeye from their modern, cheesy slasher adaptations rather than the original stories.