If the soft fork fails, Core developer Luke-Jr said they'd change the Proof of Work algorithm and possibly create an altcoin (or bitcoin if enough follow).
This is like a Russian Roullete situation. The gun's loaded, all signs point to neither side conceding. It's the cypherpunks vs the corporate miners (plus some former devs and very credible people).
There are three factions here-- Segwit2X and Core, who both want segwit, and Bitmain which does not want segwit and which has threatened a forcible hard fork into a bitcoin that has unlimited block size-- and they are even talking about doing an ICO and things like that:
https://blog.bitmain.com/en/uahf-contingency-plan-uasf-bip14...
And Luke's subsequent comment said if enough followed then it could still be called bitcoin.
Also Maxwell said he'd quit if BIP148 fails. If the soft fork fails, bitcoin will be something different.