If such features can be implemented via runtime type erasure on JVM, you can still do that on CLR - it's not like it prohibits that technique, it just doesn't use it for generics. You can even have different languages agree on how they would implement it, so that they could fully interop. With modopt/modreq, you can capture it all in metadata, as well.

It wouldn't interop with C# generics (although I don't see why it couldn't interop with C# by other, less convenient means). But if it can't be properly mapped to them when they're reified, why is there an expectation that it should? It seems to me like the gist of the argument here is that we can conflate two features into one, if only we remove all the conflicting bits of one of the features - which also happens to be the one much more broadly used at the moment. It's a strange trade-off.