IMO the biggest problem is simply that not enough devices are sold with it.
The amount of people who want to/can be arsed to/even know they can install an OS on their PC is pretty small. And even then, most that can will just stick to what they know (this is obviously part of the mindset issue that you speak of).
If mainstream devices were sold with Linux on them, it would get over that hurdle, and also get over the daunting hurdle of “ok I want to switch to Linux, what’s a good distro?” hurdle, to which people online will say everything from Debian to fucking Arch Linux.
Chromebooks (bastardised though they may be) and the Steam Deck prove that Linux isn’t unviable. People just won’t install it of their own volition.