I like this idea. I never use rendered docs, I always see them from the source code. And HTML markup adds too much noise. Markdown should be good replacement.
HTML inside of javadoc is very awkward. I hardly ever use it unless I know I'm going to be generating javadoc for a published library. Instead, I'm just already putting markdown-like formatting in the comments already.
I much prefer that to switching to Markdown. Markdown is great for plain text but it's not great for this type of content imo.
At the current trajectory of java making it to hn front page seemingly every day with yet another mosaic piece of trying to become kotlin, we can only hope for jetbrains ownership to not be easily tempted by Oracle dollars.
Java generics ended up the way they did because Java had/has a strong backward compatibility guarantee and wanted to fit into existing List/etc. interfaces and classes that work on objects. This resulted in the decision to erase the type from the runtime bytecode signature.
C# supports reified generics as a result of looking at how Java implemented generics and the problems that introduced. Microsoft were also in a position where .NET/CLR were at version 1, so made the decision to break the bytecode for version 2 in order to support reified generics.
Scala, Kotlin, and other languages have worked to follow the C# model as much as they can within the limitations of the Java bytecode/runtime.
What is missing in Markdown?
I don't even remember last time I liked new Idea feature. They're breaking my workflow with every new release. And I can't stay on old release because I need new Java support which is not backported obviously.