Sharing code between frontend and backend isn't a bad idea, yes. However, using Json for it is. And the problems and limitations with json can't be "fixed" in any way.
I don't see why serializing close to JSON is any worse than doin so to, say, S-expressions, or Algol-style syntax, or WASM.
If you aren't manually coding it, but generating it, the capabilities matter, the syntax matters a lot less.
But what reasons and are they relevant to this use? For instance, I can think of one really good reason (actually several different ones justifying different formats getting used depending on the actual use case, but they can all be summarized the same way when it comes to not using JSON): Shipping an AST between endpoints one of which is a web page and therefore has JSON as its best supported generic object serialization format isn’t the use case.
Or do you think we are at our peak?
You mean like every single idea/project/etc that ever existed before their invention?
From now on, when someone tells me something is a bad idea, I'll take it mean that I should examine it further.