The external structure of Space Mountain is easily (IMHO) the third most iconic image of Disney parks after the castle and SSE. It’s instantly recognizable by many people even those who aren’t huge Disney fans.
I’m not saying the ride will never be replaced but I would expect anything going there to retain the external architecture (ie at worse get the GMR to MMRR treatment). More likely is if they every replace it is it would be Space Mountain but “now featuring Wall-E” or such.
I think it would feature Lightyear if they add IP. I was thinking maybe they'd work in Strange World, but I dont think it's about outer space. (is that movie still even happening?)
By the way, we went on Space in October, all of us late 40s and I forgot just what a plain old fun ride it is.
I hope it never changes in my Disney going lifetime.