See I don't think we will find out. Because with so much uncertainty regarding what amounts to a hostile takeover, the ones mining the takeover attempt are putting their investments at massive risk when sticking with core is a safe way to maintain a stable economy, with or without segwit.
It may be safe technically to stick with core, but not economically, since they have failed in their leadership to scale Bitcoin and even seem to be anti on-chain scaling.
This is the reason that miners are starting to move away from them. Those miners see the economic risk as an even greater threat.
Their leadership is only about whether or not they offer a good solution for people to follow. If miners then decide that they don't want to join that solution, it's not Core's fault, nor is it a problem.
Economically, all that matters is whether the majority of the money in the Bitcoin network would feel confident when a certain solution went through. These miners that you claim are moving to BU will barely be over 50% if/when BU is actually activated because it's only rising very slowly, and therefore a hard fork would be extremely messy and
economically a bad thing.
Maybe, for a little bit. I think that is why big block miners are waiting - because they are only getting more support and they are waiting to make it a smoother transition. The trend is in favor of big blocks and that isn't going to change.
However, if we wanted to be messy we could start mining big blocks now and destroy SegWit, and it would be a costly endeavor, but it would pay off in the end.