I think several DEs could see mainstream adoption.
If the team that works on Cinnamon got a little bit more manpower and were able to implement larger changes such as adopting Wayland, I think they’d have a chance. Wouldn’t hurt to make the default theme a bit nicer too. I think the main thorn in Cinnamon’s side is the development pace and the fact that it would probably be viewed by the average person on the street as a weird Windows clone.
Plasma’s largest obstacle to mainstream adoption is bugs and instability, but in fairness it has improved a lot over the past couple of years. Seriously, compare 5.27 to any Plasma 4 release or any Plasma 5 release before like 5.16 - it’s night and day. Kwin still crashes and takes all your programs down with it, though. That’s a showstopper, but will be fixed in Plasma 6.
Speaking of Plasma 6, the fact they keep pushing it back probably means they want it stable from the beginning. KDE are doing a good job putting the “KDE is buggy” statement to bed.
I guess I agree that Gnome as it stands is the most appropriate for widespread adoption. It’s extremely polished and beautiful, it has comparatively decent accessibility features, it’s extremely stable despite being a frequently updating distro, it has amazing gesture support (better than MacOS even, imo), it’s decent in terms of touch support, the GTK4/Libadwaita app ecosystem is healthy, etc. but it’s not completely without issues.
Unfortunately this is all academic though until big laptop OEMs start actively pushing for Linux on their devices.