> How many different levels of interpretation and compilation does V8 do?

This part is key to the OPs objection. They’re saying that V8 is a very capable optimizer (which it is), so why can’t it optimize the things React Compiler optimizes? It’s a fair question.

> compilation steps don't cause cognitive load if they work

Personally I disagree. Any source code transformation adds cognitive load because my original source code now doesn’t match the source in the browser. Source maps are the traditional answer to that but if React is rewriting logic (like getting rid of arrays) you’re not going to be able to maintain a 1:1 source map.