I wasn't talking about altcoin incentives. But it's a valid question, i.e. what is or would be people's motivation for using a sidechain at all.

If it was properly trustless in terms of pegging in and out, and as long as the peg-out (*) didn't take a ridiculous amount of time, it'd be easy to argue that even if you didn't want *any* feature that doesn't exist on BTC, you at least would enjoy having much lower fees (however this assumes the sidechain gets any network effect, so you can actually use it ...).

(*) There's this counter argument that even with slow peg-out it's fine because you can use swaps to go in and out. That argument is far from a slam dunk; swaps can be costly because you need the liquidity to do them, from somewhere.