The vast majority of people that buy Macs for the ecosystem aren't going to switch to Linux. That market will remain untouched. Outside of a few gamers who might want to put up with the x86-to-ARM translation layer and (for most A to AAA games) Proton to run some non-Mac games. And even they'll probably still dual-boot.

There's a portion of another market: people who want to run Linux and want a powerful laptop who buy x86 Laptops right now. Apple could expend very little relative effort while offering no official support by helping Asahi get that to a first class platform. They won't capture them in the ecosystem (and they never would have) but will still benefit from hardware sales to them.

Obviously, if they sold their hardware at a loss and subsidized that with ecosystem capture that would be a non-starter. But from everything we know, the hardware itself is very profitable.