So back to the OP, I should have stated this:
Altcoins, in the OPs intention, are only created by hardforks. Softforks can't make altcoins two surviving chains.
SegWit would only be an "altcoin" if there was a hardfork with two surviving chains. and segwit was minority
whereby BU and a flock of other implementations of consensus (classic, xt, and other adjustable blocksize PEER imps) become bitcoin
due to having majority of hash, nodes and merchant acceptance
core could kill off its minority chain and then just add a few lines to be dynamic and join the PEER network of many implementation
BU would only be an "altcoin" if there was a hardfork with two surviving chains. and BU was minority
whereby segwit and a flock of other implementations of consensus (classic, xt, and other imps running as downstream second TIER) become bitcoin
due to having majority of hash, nodes and merchant acceptance
BU could kill off its minority chain without any code changes can just join segwit as a downstream second tier of many implementations
BU could kill off its minority chain and can with segwit code additions join segwit as a upstream first tier with core
i removed this line:
If BU survives and the minority chain dies off, BU transforms from altcoin into "Bitcoin".because if BU had majority. its already bitcoin along with the other implementations (where segwit is the altcoin or just dead)
theres still some irregularities even with the FTFY summary. due to when/if core triggers their bip9/UASF/PoW change, and which markets treat it as such. and as highlighted if there remained 2 chains or not. or the opposing implementations give in to join the peer or tier network
but its close enough