Seems like the biggest downside would be that sharing code becomes much harder. Currently it is easy to share code in as small of a portion as you'd like. If the canonical representation is an AST, it opens up a lot of problems around sharing pieces of a program. This seems like a very substantial downside.

Even simply "sharing" within your own systems, like copying blocks into notes or another program, would be a lot harder. Maybe I'm not knowledgeable enough here and this wouldn't be as thorny as it seems.