Well basically that means there will be a hard fork with a replay protection or something, for sure. Coinbase, Xapo and all the NYA signers not going to move S2X chain but wait and see if S2X going to survive or not.
If the majority hash power moves to S2X and Legacy Bitcoin blockchain gets weak due low hashrate, NYA companies will have a good reason to convince their users about why they will move to S2X "to protect users money".
Unfortunately, that gives Bitmain a lot of power here. People are under the impression that Bitmain doesn't care about Segwit2x because of Bitcoin Cash. I disagree. The BCH fork was largely about pushing the ecosystem away from Core. This represents another opportunity for Bitmain to do that, by making the legacy chain the weaker chain.
It's already widely believed that most of the Chinese pools (as well as BTC.com) are just Bitmain proxies. And in private, according to Greg Maxwell, Jihan Wu has bragged that Bitmain controls a majority of the hash rate outright.
That is the first option. Second option is, let's say %50 of the hashrate is moved to S2X. So, Original Bitcoin blocks will be created more slowly, UTXO will grow up and yes network congestion will happen but I think the gap will be replaced quickly.
This may happen but in the long term, it won't sustain. The market will eventually tip one direction or the other, and miners will follow. The question is: how much effect can miners have on the market? If most miners leave the network, that may have a big psychological effect on holders/traders.