This doesn't account for those customers that aren't going to go cross-platform because they want to stay with MacOS, but do want high performance, which applies to most of Apple's higher-end desktop users.
I recall running simulations on a Power Mac G5 while my friend was runnning similar ones on his (significantly faster) AMD Linux box. I wasn't going to switch to Linux, since I didn't want to give up MacOS's much nicer UI (and MS Office), but I was not pleased that, in exchange for running MacOS, I was stuck with a slower machine.
Apple wants its customers to be happy with its products. And there are many higher-end desktop customers that, being in that category, want a computer that's highly responsive with minimum lag, which typically means a really high SC speed (relative to the software you're using). And while I haven't yet switched to AS (I'm running a 2019 i9 iMac), I gather Apple isn't there yet. That means if they do increase SC performance to a degree sufficient to be noticeable, it will contribute to customer satisfaction.
I'm not saying Apple will cater to such customers by offering higher SC speeds in the Studio and Mac Pro than in the Air. I'm just saying many who would like, and benefit from, maximal SC performance won't switch to PC's, since the tradeoff of not being able to run MacOS is too great for them.
Plus you want to account for switchers in the other direction. To expand its market, Apple needs to convert PC owners. Converting higher-end PC desktop users requires offering them something that isn't a performance downgrade.