If users could install Telegram from third-party sources, without rooting their phones, this issue wouldn't be so serious. Or if users could install from trusted friends, even. That is, if iOS apps could be shared. There's a similar issue for Android apps. But it's my understanding that it's easier, and doesn't require rooting.
But of course, this would break Apple's "we are the only app source, because you can only trust us" security model. Or maybe it's "... because we can only trust us, and we don't want increased support load".
Installing apps from third-party sources is trivial on most Android phones. It only requires turning on a setting that's off by default and exposed in the regular settings app.
On android it's very easy. Settings, developer, "allow installation from 3rd party - on".
Alternatively on my phone when I try to install an app, it'll allow me to change that setting just for that single installation and then turn it off again.
Which they do, to some extent. They're in a weird limbo right now where they're pushing updates but not updating the publicly available source, which is something I find vaguely off-putting.
But of course, this would break Apple's "we are the only app source, because you can only trust us" security model. Or maybe it's "... because we can only trust us, and we don't want increased support load".