A lot of FOSS developers make apps largely for their own use. When you’re not a business you don’t really care about market share as much. It’s a tiny fraction of android users who even use F-Droid in the first place. I would guess that many of these users are already on degoogled android but I don’t have numbers. But I don’t think there’s a need to be so so pessimistic.
I suspect that the OSS devs who really care about getting their app to a more mainstream audience already prioritize the google play store or don’t use F-Droid at all. Thinking about Signal, OnlyOffice, Proton Mail for example.
Of course, these policy changes from Google are also coming during a time when Google and other manufacturers are continuing to make it harder to flash degoogled android in the first place. But GrapheneOS seems to be actively working on getting around these types of restrictions too.