My biggest problem is security updates.

The "x years of upgrades" model is okay when it's for an app, where you can just keep using it with the old feature set and no harm is done.

But Unraid isn't an app, it's a whole operating system.

With this new licensing model, over time we will see many people sticking with old versions because they dont want to pay to renew - and then what happens when critical security vulnerabilities are found?

The question was already asked on the Unraid forum thread, and the answer from them on whether they would provide security updates for non-latest versions was basically "we don't know" - due to how much effort they would need to spend to individually fix all those old versions, and the team size it would require.

It's going to be a nightmare.

Any user who cares about good security practice is effectively going to be forced to pay to renew, because the alternative will be to leave yourself potentially vulnerable.