If miners soft fork, and you do not, and the change only affects them, and not block size or anything else on main or having to do with nodes, blocks or transactions , how have you changed anything ? Not everyone and not every miner is on the same version of core right now. Bip300 in particular does not give a fuck about nodes.
Its opt-in, affects no one who doesn't want to participate in sidechains , and is a minimal change. Far less than taproot and segwit.