Perhaps the key is for them to work closely with Valve to make Proton work well on macOS, which would also neatly deal with the 32-bit issue because WINE/CrossOver on macOS handles 32-bit Windows binaries just fine despite macOS having dropped support for 32-bit Mac binaries ages ago.
This however seems unlikely because even setting aside the issue of how Apple might see doing this as pushing users to a competing marketplace, it also cedes some amount of the Mac's functionality and thus marketability to a third party which Apple has never been keen on.