On the other hand, the very reason I use Linux is that I get more freedom, including the freedom to choose my window manager.

I don't care so much about Linux Desktop becoming mainstream. Probably it would make it look more like those OSes I like less. I don't really get those comments I regularly see where people go "if you don't make it look like Windows, people won't migrate to your distro". If I wanted Windows, I would use Windows. And I don't want people who want Windows to come to my distro.

Meanwhile Valve failed to convince games industry to care about GNU/Linux as platform, and has to make use of Proton to make SteamOS relevant, even though PlayStation and Android are very much Linux like on their technology stack.

I think that Proton works pretty well. I am very impressed by Valve's effort there and the amount of games that are now available on Linux.

But I don't think it's related to the Desktop environment, is it? Somehow I feel like many people who complain about Linux not being "mainstream material" want Windows, but free and without ads/trackers.

I don't want Windows, I want freedom. Freedom to choose my init system, my window manager, my filesystem, my terminal, hell even my libc.

The thing is, I don't think it can happen with a mainstream system. The industry won't care about supporting Linux unless there is critical mass, and in order to have critical mass you have to onboard all those people who want Windows (but free), not Linux. Already now, too many projects only work with the latest Ubuntu and assume systemd and some specific dependencies.

If Ubuntu were to become mainstream, instead of having Windows, macOS and Linux I fear we would have Windows, macOS, Ubuntu and Linux. Not sure it would change anything for me.