But Debian only gives LTS for 5 years. Ubuntu gives you free LTS for 10 years if you sign up for the free ubuntu pro account. That's what the "nagging" is for. If you don't sign up, you still get 5 years like Debian.
It's not about length of support (I generally upgrade from LTS to LTS versions of software), it's about not wanting to be advertised to. (I had been using Ubuntu on my server for over a decade, and was always irritated about those MOTDs, even though I knew how to delete them.) I also moved away from Ubuntu for other reasons, like them pushing snap packages for browsers (and probably others I was unaware of).