the LTS model is pretty common. Even if .net 22 LTS is only bug fixes for .net 20 LTS, maintaining multiple releases of the same library is expensive. The current LTS model requires maintaining 2 current releases and one future release. As it becomes more mature, I'd expect fewer code changes between releases. Maybe even auto update in the build tools.
as security is now an issue, you should be upgrading you code regularly, nuget packages especially. If you do not have a release cycle for security fixes you should.
Most likely .net core will get long term support when it matures like .net 4.* and is replaced by another framework.