Alright, fair enough. So we have:
Forces an approach: I don't necessarily agree with this argument because being down in percent also forces you to approach. It's an inherent part of the game we have to deal with, or play a different one. That said, the stage does put a player at an immediate disadvantage upon starting the match, being spawn point. This is random, so it is unfair for it to be considered okay.
Over-centralizes towards camping and chaingrabbing: This one needs to be more specific, as currently it doesn't sound any different than what we already accept about Rainbow Cruise. I'm not disagreeing, but this doesn't pass without something to back it up.
Permanent Walkoff: This is false, as there are times where a pit in the ground or a pipe or the blocks near the flag are present. There is, however, a walkoff that appears often. This is just arguing semantics of permanent vs. practically permanent, but I'm covering it so that Jebus doesn't have to.
Walkoffs and/or walls almost continuously: This one is true. The walls are almost a non-issue because they move along with the stage and don't allow any characters to use them in a way that breaks them. Walls are banned regularly for enabling wall infinites and other shenanigans that require the wall to remain in place. The walkoffs are more of an issue. While not constant, they are present very often and create a very high-risk/high-reward scenario that detracts from the gameplay.
Temporary Cave of Life scenarios: Since they are temporary, they are not a problem. The scenarios that arise are nowhere nears as bad as Luigi's Mansion can be, and they are over and done with in due time.
All this said, this stage should remain banned. Jebus, please take your campaign to ban Rainbow Cruise to other places. There is no need to advocate for a clearly un-competitive stage in order to ban RC.