If the goals of future game development are pro-consumer and developer-centric like platform agnosticism (or, preventing artificial platform monopolies like, eg- GFWL) and simplifying development, then we're doing way better than good enough.
We've learned over the past decade that publishing Linux native games isn't as feasible or valuable because supporting them smothers smaller and independent developers in distro-specific bugs, robbing them of time and resources they could be putting towards developing content, spending time with family, or planning their next projects. There's no reason to publish natively as long as you can develop with Proton/SteamPlay in mind, and instead of every indie dev needing to fumble through learning Linux support, it shifts the burden to specialized developers (like Valve) who can more effectively contribute to Proton instead.
Linux native support is almost an anti-criteria for me today, because I'd much rather troubleshoot a known-working build with all the features enabled (eg- multiplayer works with other "Windows" versions) instead of a build that lags behind in updates and may never be more than a perpetual WIP. Proton and tools like DXVK and D3DVK that can be ported anywhere exist to circumvent that, and that's pretty amazing when you think about it.
The problem isn't the compatibility, it's the reliability. You can't expect every game to work under Proton. It's always a gamble.
If there was a guarantee that every game works then it would be indistinguishable from excellent first party support. The reason why it's only good enough is that you have to sacrifice the titles that don't work.
On the other hand, the windows demo probably works, because wine still understands old windows apis and current linux apis.