Let's be very clear about a few basic facts:
Every single thing on this planet changes, upgrades, whatever name it. Why not accept BU?
BU is a copy of Bitcoin as it was
two years ago, with some changes on top of that. That is not an upgrade
Bitcoin upgrades with every new version. Segwit is an upgrade to the protocol itself. Throwing away two years work is not a good idea even if you think the development team must be replaced.
BU is bitcoin. It's BU vs Core, not BU vs BTC.
No, it's not. Why would you even say that? There are a bunch of competing Bitcoin implementations. And then there is BU.
It's (BU) vs (Bitcoin Core + btcd + libbitcoin + whatever custom stuff the big exchanges use).
Every major Bitcoin implementation must support a new feature before we can use it in a safe way so no one loses their money.
If the Core implements big block, there is no need for the Unlimited. I think Core should be more open.
Core is an open development project. Anyone can suggest improvement, and if you pass peer review and convince other developers that the idea holds water, you are free to implement it. BU wants to replace that with a team of salaried developers. Right now it looks like
two salaried developers and
zero salaried QA.
Some big blockers supports BU because of the possibility that it might lead to bigger blocks (which nobody has shown how that would work in practice), but the miners support it because they think they have (more) leverage over the two salaried developers in question than they have over the current open source project.
So, no, bigger blocks in Bitcoin Core would perhaps dissuade a handful of people to support BU but the miner support would still be there. They would still want to rip out the last two years of work and replace it with their own developments. This is increasingly clear since Bitcoin will unlock a blocksize increase with segwit, but now BU argues against that increase because it was made by "tainted" developers. That must take an impressive amount of cognitive dissonance for a big blocker.