That said, as others have pointed out, the vim developers can't really be blamed here as it's just a very hard problem. There isn't a common convention, and vim already goes above and beyond to try and educate users on how to get out. People ending up on this SO question just seem unwilling to read.
Sounds like a distro problem.
macOS is another potential culprit - can't remember the default there.
The problem vim has is that its UI is entirely organized around modes, which makes the "people are terrible at using modal interfaces" problem a hard one for it to cope with.
Not to mention your OS is full of modal interfaces. "Why can't I type?" "You have a menu open." "Why can't I type when my cursor is over this window?" "You have a different window as your focus." "Why doesn't space pause this YouTube video?" "Because play wasn't the last button clicked." "WHY ALL CAPS?" "Check your capslock key."
None of these are examples of modes, at least not as the term is used in the context of user interfaces. (See https://en.wikipedia.org/wiki/Mode_(computer_interface)#Defi... for more information.) Modes hijack the entire behavior of the application until the user switches out of them. Windows and menus aren't modal unless they block all other input until the user deals with them.
On the other hand your last example, the CAPS LOCK key, actually is modal; once it's engaged, everything you input is going to be capitalized until you disengage it. You will also notice that the CAPS LOCK key is a common source of user problems in tasks where capitalization is significant, like entering passwords; and that for this reason there are frequent calls to remove it from the standard keyboard layout.
Ditto the multi window scenario. You have to actively change focus between windows (often between windows in the same application), until which time the application doesn't respond as you expect it to.
Another great example: spreadsheets. You're in one of two modes, editing a cell or moving around cells. Often without great feedback of which mode you're in.
Modal interfaces are everywhere.
People "terrible at using modal interfaces" should just use another editor.
The problem might lie in other applications indirectly launching VIM, taking for granted the users' familiarity with it.
They're often not intuitive, but intuitiveness is not the be-all and end-all of interfaces. It's a trade-off.
You may as well expect Porsche to move the 911's engine to the front.
The vast majority of people don't use vim, so this "solution" has already been widely adopted. (Which in turn causes people who like vim to periodically wonder why more people don't use it.)
The problem comes because there are scenarios where you don't have a choice as to which editor to use; if you're shelling into a server with limited privileges, for instance, vi/vim may be the only even remotely modern editor available. So lots of people find themselves forced to use it, and these are the people for whom the Stack Overflow thread is useful.
Do we need to spoon-feed you people the solution for all of your problems?
Use SSHFS to mount your remote file system, and edit your remote files with your fancy, LOCAL, non-vim editor.
Like... nano? Or pico? All quite broadly available, and easy as dirt to use. You can even use them to work with `visudo`, one of the few times where I could imagine you don't have a choice.
Also, if you can edit it remotely over SSH, you can scp it in two directions and use your own favorite editor (assuming that editor doesn't already have something like network editing already built in; most do).
If you "don't have a choice", you're not looking hard enough.
That doesn't sound like a reason to change vim, it sounds like a reason to change the configuration of the server to something saner. pico/nano makes sense to me, if the expected users are the kinds that wouldn't know how to exit vim or emacs.
That said, any server admin who for whatever reason allows people to SSH in (shared hosting maybe, university, file sharing) should have nano installed. Anything else is just cruel!:wq
They probably learned how to do that in a Stack Overflow thread as well. No competence necessary.
The problem is you do crontab -e or something similar on a new or foreign system and get into vim and are now stuck.
Not everything has to be easy to use, some things benefit from not being obvious.