LTS doesn't mean it's immune to bugs or security vulnerabilities. It just means that the major release is updated and supported longer - but you still need to be able to apply patches and security fixes to that major release. Yes, it's easier to go from 1.20.1 to 1.20.5 than to 1.21, because there's less chance of breakage and less things that will change, but the process is pretty much the same - check for breaking changes, read changelogs, apply everything. The risk is less, might be slightly faster, but fundamentally, it's the same process. If the process is too heavy and takes you too long, having it be slightly faster won't be a gamechanger.
So LTS brings slight advantages to the operator, while adding potentially significant complexity to the developer (generally backporting fixes into years old versions isn't fun).
The specific proposed LTS falvour is also hardcore, without an upgrade path to the next LTS. The exact type of org that needs an LTS will be extremely reluctant to having to redo everything, 2 years later, with potentially drastic breaking changes making that change very hard.