I think you can totally start a .NET Runtime for a plugin. There are windows explorer plugins written in .NET - I know because Raymond Chen analyzed how they broke things :)
If you have freedom then you have the the freedom to make mistakes.
In Windows globals are tightly scoped to a DLL and cannot accidentally cross over, so multiple python interpreters are no problem if the python interpreter is statically linked in each python plugin.
It’s just much easier for an editor to manage a single interface for plugins over having to host a bunch of very large runtimes.
Also, I doubt that you would have a very great time running both .NET and JVM in the same process. For one, both of them install signal handlers, and both of them do funky things with threads.
If you have freedom then you have the the freedom to make mistakes. In Windows globals are tightly scoped to a DLL and cannot accidentally cross over, so multiple python interpreters are no problem if the python interpreter is statically linked in each python plugin.