Quoted from reddit:
"BIP-91 lock-in does not guarantee segwit will be activated unless BIP-91 is actually enforced.
Soft-forks are ultimately enforced by users, not miners, and right now practically no users are running BIP-91 enforcing nodes like segsignal. So a majority of miners can reneg on BIP-91, fail to activate segwit, and get away without any long-term consequences because the BIP-91 invalid blocks they would be producing are considered to be perfectly valid by the vast majority of users' nodes.
Will this happen? It's notable that even though BIP-91 just activated - something which is supposed to signal clear intent to activate segwit - a bit less than a majority of miners are actually signaling segwit. While no smoking gun, it's a sign that we may be in a situation where a majority of miners do not intend to actually activate segwit.
I don't think this is the most likely outcome - there's other pressures like social that probably would serve to enforce BIP-91 - but I'd give it significant odds of happening. Maybe 25%?" - Peter Todd
What are you thoughts on this? I'm a little confused because everyone I've talked to says if it's locked in then it's going to be unlikely to fail as nobody wants to be the one pool that does a 180