An interesting anecdote for you about just how much an exe/dmg equivalent may be the missing link to mass adoption:
My older brother needed a new laptop. A little bit of background about him, he is not technically inclined at all and doesn't want to be. He uses Siri to operate things like alarms and reminders on his iPhone because he doesn't know how to use them. I even offered to show him, he doesn't want to know. This is someone for who an alarm clock app is too technical to get involved in.
So anyway, he needed a new laptop, and I of course had a fair collection of Thinkpads so gave him one of them; loaded up with Fedora (Gnome as the DE). I installed the basics he uses, pretty much just Chrome and the Spotify Desktop app.
He's been using that for a few years now and far less than having difficulty with it, he actually loves it, and says (I suppose mostly due to the speed of it) that he wouldn't want to go back to Windows.
Now obviously Flatpak can install everything he uses there, but I think for most people, that being an option rather than a requirement is scary. Windows doesn't have the option of using .exe, thats just how you install things, like it or not (*yes I know there's winget and whatnot, but you've gotta be pretty in the weeds to even be aware that exists). And if you want to install some non-free software, you can find yourself in a difficult position.
Clearly from my brothers case, Linux is now at a point where anyone can use it, I do think the last missing link is a unified executable/installer, that works across distros (in his mind, he's not using Fedora, he's using linux) and works in the same way as exe and dmg. And not just as another option either (see xkcd 927), but as a core requirement of shipping on Linux, and that's a far more difficult problem to solve.