If I remember correctly, it was the first whatever-to-JS transpiler. But it opened the floodgates and "do everything in one language, bridge the gap transparently" has been tried several times since then.

And even before that, actually! Before web apps were even a thing, we had DCOM and CORBA and some other similar but less-known frameworks that tried to make OOP in general network-transparent. It worked in principle - you could have distributed object graphs in pretty much arbitrary configurations, going back and forth as needed to capture the semantics. It failed in practice because every time you have a network gap you get a slew of potential issues that just don't exist without it (simply put, your connection and/or the other party may suddenly go away).

FWIW I'm not saying that single-language specifically is a bad idea. It's specifically the notion that you can treat a distributed app as a monolithic thing without clearly marked internal boundaries where the network gap is, that fails in real world. But if you expose the gap then you still need to deal with impedance mismatch - e.g. nice your object oriented API no longer works because the object graph can't span the gap, so you need a more procedural (read: REST-style) API with serialization etc.

So, this is the point where you basically want a language designed from grounds up with message passing in mind. Blazor, but for something like Erlang or Elixir, perhaps?