Would miner prefer to have more reward in future, rather than now?
If the protocol prevented them from grabbing that "burnt" part of the transaction fee, then they would not have this option. Or maybe you refer to the possibility that miners could reject that soft/hardfork? I think in this case everything would depend on the signalled support from economic nodes.
Yeah, i imply current miner would reject such soft/hardfork. As for readiness signal from node, i doubt it's enough to pressure miners accept such change.
Merged mining also can be done with Bitcoin layers rather than altcoin. On theory, Bitcoiner would use of one of those Bitcoin layers to make TX with lower fee and faster confirmation. But Rootstock proved it's unpopular option among Bitcoiner.
I think RSK has also the problem that they are considered a centralized entity (instead of a real decentralized sidechain) due to their federation model, and thus they aren't that popular.
Yeah, RSK is just example. But looking at
https://www.bitcoinlayers.org/, there's little alternative that can give miner some more income while not being more centralized/require more trust than RSK.