Well in theory it will be an alt as it'll be a new coin, but it'll be based on a variant of Bitcoin and it'll split from Bitcoin so the majority of mining power supporting BU might result in this alt becoming more popular, thus making the "original" Bitcoin struggle.
Interestingly, I could see the old Bitcoin adopting SegWit and potentially becoming even more popular than BTU afterwards, so it'll be interesting to see how it plays out.
Bitcoin Unlimited is a solution which is closer to the original white paper. Core's solution is introducing a two-tiered network system with the original bitcoin layer used as a settlement layer, and users using banking style LN off-chain solutions. The latter sounds more like an alt to me.
Follow the influence trail:
core < blockstream < DCG < financial sector investors
Banks do not like bitcoin because they cannot control it. If people value bitcoin, for its trustless transaction nature, they should consider if they are sleep walking into a situation where the banks are allowed to control it. Do you want to hand it to them on a plate?
Note: I went into this discussion thinking segwit was a good idea. I now severely have my doubts about the direction it is taking bitcoin in.
