EDIT: Everyone here knows how to type "sudo apt install thunderbird." People are failing to install bleeding edge software. This shouldn't be surprising or upsetting.
Package managers exist for a reason. Flatpack, snap etc all exist. You can't just hand wave real complexity away by yelling "usability!" without having fiat power over an incredibly diverse ecosystem.
The lowest common denominator for installing software on Linux distributions is well below what an incurious user will put up with. If you don't want to understand that kind of low level tinkering, you have to wait for whatever ecosystem you bought into to catch up.
That might mean moving back to Apple's curated experience. That's fine; so long as Linux itself isn't a curated walled garden, it'll never be as seamless in every instance, and that's not for everyone.
Addressed by whom, exactly? "The Linux community" is busy addressing the things they care about. Nobody is telling anybody to use Linux if it doesn't work for their needs.
Linux is immensely useful to me and everyone else who uses it. It doesn't really bother me that my mom doesn't use it; she is perfectly happy with Windows. If someone made a Linux distro that was exactly as polished and easy-to-use for non-technical people as macOS or Windows is, it would not affect me at all.
Who is this strawman entity who "wants Linux to be mainstream" ? Does anyone really care? Certainly my mom doesn't, and as far as I can tell, most Linux users who are happy with it don't either.
Furthermore, it's quite easy to understand why this hasn't happened, as doing that would cost huge amounts of money and there is no incentive structure that exists to raise it.
I think it's basic human desire for things to be relatable, but more than that there are those who experience the tyranny of 'modern' life - if you interface with the world, it is perhaps natural to want to do so simply, constantly switching contexts is not productive.
I suppose, I do really care, because every time I am forced to use a system which I don't really want to use, is another time I don't really like the world I live in, I am resentful, and ultimately I really want to not care at all about the device in question.
So, for a wide range of devices I interact with, I view them as trash, because I don't care about trash either, except to remove it from my home on a regular basis.
There are others like me, I know, and there are others in similar positions but not about linux I know as well, e.g. anyone who has ever read a good book and wishes the author were more well read, and simultaneously is affronted by the cheap dime novellas they encounter while e.g. browsing a department store.
"Furthermore, it's quite easy to understand why this hasn't happened, as doing that would cost huge amounts of money and there is no incentive structure that exists to raise it."
There isn't, yet. There are various companies which have sought to bring it there. To use the (perhaps bad) author metaphor, we haven't had our JK Rowling moment yet.
There have been pushes, e.g. Ubuntu, Redhat, but so far market forces have conspired against us, not necessarily because no one cares or no attempts have been made.
Linux based OSes will never be popular and have the same freedom they have. The principles the developers have that make it free are the same ones that create these "usability problems" which make it so unpopular.
Legos, Jeeps, Cooking. HTML, Minecraft, some real-world, software options.
"Usability" can be something you buy on top of the simple system, or it can be free, depending. These aren't diametrically opposite properties of systems.
The ones doing desktop environments, UIs, package managers, etc. They thrive for mass adoption.
Developers that want their applications cross-compatible and painless for the users.
People that cannot (or are not willing to) pay for a Windows license.
The ones waiting for the "year of Linux Desktop".
> a Linux distro that was exactly as polished and easy-to-use for non-technical people as macOS or Windows
While I truly appreciate the efforts, the problem is that the above is the intention, but it's kind-of half baked right now.
For example, having to search online, open a console and editing files for simple stuff like regulating the speed of the mouse wheel.
I doubt it. I have never seen anyone in the Ubuntu community for example claiming that their goal is the "mass adoption" of Ubuntu. That's, obviously, an impossible pipe dream. Can you provide recent examples?
I'll just use common sense and say that all the evident effort that is put into Desktop environments and user applications is not just to keep their (tiny) user base happy or to be used less.
In particular, I hate that I need to have a Windows machine for gaming in addition to a Unix machine for everything else. If Linux were a substantial competitor on the desktop, more developers would feel compelled to port their games to Linux, and I wouldn't have to make that choice.