But while legacy (ie. non-SegWit) nodes would accept such a transaction, the majority of the network -- and thus the canonical BTC blockchain -- would reject it.
Most importantly (to me), MY NODE will reject it. It won't matter how much hash power they have. They could mine 99% of the blocks, and my node will still reject it because it will STILL be an invalid block with an invalid transaction.
The only people that will be effected will be those foolish enough to still be running a non-segwit enabled wallet on the SegWit network.
It doesn't matter if your node will reject it, I think that you forgot that miners have complete power and if you are not mining there is not much you can do. Miners are the ones who control the network I mean we should say mining pools as mining solo is not what we want. Anyway I think if this happens however we will see that BTC price will dump since people will not have faith in it due to mining pools living to their own terms and thus it would drastically decrease profit from miners.
The real question here is are they really ready to go that far to hurt BTC in a way where they will lose potential profit? I think not.