By supporting Proton, they are guaranteeing that modern and retro Windows games will be playable on Linux far into the future. Trying to get the next Call of Duty to support Linux natively is, quite literally, a waste of everyone's time that could possibly be involved in the process. I cannot see a single salient reason why Linux users would want developers to release a proprietary, undersupported and easily broken native build when translation can be updated and modified to support practically any runtime.