We don't have to imagine what Linux gaming would be like without Proton.

- CD Projekt Red: released Witcher 2 on Linux, didn't for Witcher 3.

- iD Software: released Doom 3 on Linux, didn't for Doom (2016) or Doom Eternal.

- Epic Games: released Unreal Tournament 2004 on Linux, but didn't for Unreal Tournament 3 or Fortnite. (A Linux port was being worked on for UT3, but it ended up getting cancelled.)

- Larian Studios: released Linux version of Divinity: Original Sin, didn't for Divinity: Original Sin 2 or Baldur's Gate 3

Many studios over the years have made native Linux versions, and many studios stopped because the cost of porting exceeded the revenue it generated. Proton didn't exist when Unreal Tournament 3, Witcher 3, Doom (2016), or Divinity: Original Sin 2 released, so Proton wasn't the reason those studios stopped developing Linux titles -- they stopped because it made no financial sense to continue to make them.

Now, with Proton, 79% of the top 1000 games on Steam are gold or platinum rated on ProtonDB. If you're fine with minor issues, 88% are silver rated or better. For the Steam Deck in particular, there are 5,500 verified games, and 16,526 verified or playable games. So I'd argue Proton is doing quite a lot for people gaming on GNU/Linux machines, since they now have access to a solid majority of the top 1000 games on Steam, both on a Linux desktop and on a handheld.