Windows & Linux users will still be able to use it. Most popular VPN services seem to develop their own custom desktop clients (they do this for OpenVPN); they will definitely contribute to Wireguard, but I'm not sure that they will contribute much to the desktop-specific parts of the "official" apps.
Edit: I should add that there is another cost/benefit assessment here: if Wireguard developers continue to appease Apple, Apple will continue to make life difficult for them as there will be no pressure for it to behave better.
You can't just look at the clients, you have to consider a VPN protocol holistically. Do you think that most VPN installations are going to run two different gateways side-by-side so that they can provide Protocol X for Platforms A and B, and Protocol Y for Platforms C and D? (Even worse: X for just Platform A, and Y for literally every other user.)
Edit: I should add that there is another cost/benefit assessment here: if Wireguard developers continue to appease Apple, Apple will continue to make life difficult for them as there will be no pressure for it to behave better.