There would be little incentive to port games to Linux without an existing gamer pool which can buy them, and there would be no gamer pool without an initial range of games...
The only practical way to break that vicious circle was the emulation option. Linux gamers play on emulation, but appreciate native ports and form the (initial) buyer pool for Linux games.
So Proton and co are the only reason there's any viable Linux gaming at all. Without Proton, every Linux gamer would have had a Windows install and play with that alone.