With drivechains, Bitcoin users can't opt out. That's the difference. Drivechains are soft forked into the consensus by miners.
With consensus reached, what would be bad about that? Segwit was soft forked into consensus with the backing of full nodes behnd it.
I can see the analogy you're making, but it's not entirely accurate. Segwit didn't fork a separate protocol into the consensus. I have doubts that Core would merge something like that, especially given the security trade-offs of a drivechain. Segwit was much less contentious than that.
OK. I'm still learning/reading more about Drivechain, and why Paul Sztorc believes strongly in that the trade-offs are worth it.
I'm not particularly comfortable with the precedent of miners forking in sidechains, on which users blindly trust those miners.
But full nodes secure the network, not miners.
I think economically important drivechains may skew Bitcoin's mining incentives. I'm not sure. I'd like to see more convincing game theory suggesting drivechains are a good idea before considering enforcing them at the consensus level, that much is certain.
Why? Mining on the base layer could be worth more because it supports all side-chains. Plus if a side-chain has value, wouldn't the game-theory/incentive-structure be the same as in Bitcoin?