I think that requiring certain versions of things is necessary for many things, but not extensions. I'm on my third Flarum site's upgrade and it's fairly cumbersome because I have to remove incompatible extensions and then probably due to my version of composer and hosting environment I have to do other workarounds. If things don't update in exactly the right order you're stuck until you remove certain extensions. Even if some of the extensions are in fact compatible, if the update fails in the middle you might be left removing extensions which have compatible versions out there because for whatever reason they won't update.
So, my proposal is that Flarum shouldn't be as strict when it comes to extension versions. Let the core update no matter what. If there are errors, then the webmaster can troubleshoot which extension is causing it by deactivating and reactivating one at a time, like you have to do anyway.
Isn't a lot of popular software like this? You can update the core, and then if an extension you have stops working you remove it. But, you don't have to do all kinds of extra composer commands. And, since the new admin dashboard has those dots next to extensions, a certain color could mean, warning, do not activate this extension, it isn't compatible.