I don't think anything will happen to Splash Mountain.
1) All the people begging for Zootopia over Splash Mountain could probably fit in the same room. And it could be a small room. It's really not many people.
2) Even if GOTG ToT happens against all odds, Splash still wouldn't because that would require a mountain at MK to be closed for an extended period of time. They won't even close a practically falling apart Space Mountain to make it, y'know, not falling apart, so we don't need to worry about anything happen to this other mountain.