There are so many features that .Net 5+ brings to the table. Even if features aren’t important the performance improvements you get with the newer versions should be enough to justify moving to it.
I agree the support side is annoying but honestly the support side is really just “security” fixes with security being a very hard thing to describe here and gives MS a lot of wiggle room to not actually support it.
Yes, there is other nice language features but obviously 15 years of Framework code base has probably put up guard rails around those sharp edges.
My point still stands, I can't imagine most companies green lighting .Net Framework -> Core conversion if they can't switch to Linux. If you are stuck on Windows, you have probably developed all the tooling to deal with Windows so it's all sunk costs.
Getting access to things like Kestrel (breaking out of IIS jail) is way more critical. Also, self contained deployments mean you can stop shipping magical blessed machine images around. It's not even about the performance. It's about having technology that doesn't actively hate you.
Also they aren't the only ones, same applies on the Apple ecosystem, only those around Mac OS and OS X early days actually manage to find sensible documentation.