Installing Debian is not an alternative to the 10-12 year Ubuntu LTS support because Debian doesn’t offer that kind of support. Also as the sibling noted, Ubuntu Pro isn’t needed to get the same support you’re getting from Debian. Ubuntu Pro provides additional support that you don’t get from Debian throughout the support lifespan.
BTW, not offering 10-12 years of support is totally reasonable for a community distribution. I don’t expect volunteers to be backporting fixes for packages built 12 years ago.