Linux Mint nowadays supports release upgrades, but you have to follow their blog to know when a new major Mint release is out and you have to manually install mintupgrade and do the upgrade.

So it is definitely not caused by technical constraints, as Mint has implemented the difficult part (providing and testing an upgrade path) already. Notifying the user about a new release upgrade shouldn't be too difficult? E. g. in the most simple form you could probably preinstall a package that does nothing at first, but receives an update once the next Mint release is out to send a notification to the user to inform about a new Mint release.

When it comes to elementary OS, I think they could support in-place upgrades, as they properly use metapackages (unlike Mint, which marks most packages as manually installed and doesn't really utilise automatically installed packages and metapackages in a way that you would expect on a Ubuntu-based distro), but they probably don't want to allocate / don't have the resources to test an official upgrade path.

But again, I don't understand why it is so difficult for elementary OS to at least provide a simple notification to the user that a new version is out. Even if the users have to reinstall, it is critical to inform them that their OS is about to become end of life. You know, people do things like online banking on their computers ...

It's the first thing I check with every distribution and if it doesn't have an EOL / upgrade notification, it is immediately out.