Bitpay and Coinbase aren't enough to keep it going, alternative to both have existed for years.
There are other alternatives, but that requires:
-Users to only buy from merchants which are using an alternative payment gateway that accepts the earlier chain
-Merchants to actively switch their payment gateway to a new one
Ultimately, if a user wants to buy from a merchant, they will just buy from them. It's unlikely that they will go to the lengths of actively avoiding a huge amount of merchants out of opposition to the SegWit2x chain.
relative to "who's calling what an alt coin" It aint me babe. Its (assuming miner/exchange cohesion continues) Coinbase, Gemini, Bitpay and on and on that have tons of real customers who will be calling Bitcoin-from-segwit2x Bitcoin/BTC and to use their services a Core supported coin won't be that.
It seems likely that users (and companies) will just call whichever one they prefer Bitcoin, and that there won't be a chain which everyone just simultaneously agrees is Bitcoin because the support for each chain won't be so overwhelming that the other becomes less relevant.