Curious... do you think that the world has moved on from the "download from website and run installer" model? Obviously that has serious drawbacks from a security perspective, but up until less than a decade ago, that was the only way to get software on Windows and macOS, and they were perfectly mainstream.
I think the majority of users out there don't understand sandboxing or permissions models or why they are useful. While I think those are the users that probably benefit from them the most, it doesn't follow that we need these things before average users will embrace a particular platform.
> A lot of people are mad at Canonical for not open sourcing the backend but no one seems to be offering to build one.
You mention Flatpak and then two sentences later claim this?
I think maybe you're just missing the point. I have been using Linux as my daily driver for a good two decades now. I don't care one bit about Linux being a mainstream desktop distro. Sure, if it was, new (and even not-so-new) hardware would get better support, and that would be a win. But getting that is not a fair trade off if what we have to accept in return is a closed-source, walled-garden software delivery model. Hard pass, no thanks.
For the most part, the only people who really care about "the year of the Linux desktop" are people trying to build a business around desktop Linux. Those aren't the sorts of people I want making decisions like this, but, unfortunately, they're often the kinds of people who have the ability to force these things on the community.