For what it's worth, most people already are doing this! Some of the subagents in Claude Code (Explore, I think even compaction) default to Haiku and then you have to manually overwrite it with an env variable if you want to change it.

Imagine the quality of life upgrade of getting compaction down to a few second blip, or the "Explore" going 20 times faster! As these models get better, it will be super exciting!