QUIC does a lot more than "the one for TCP". While I also believe that modern TCP consists of more than just one RFC (which you already hinted at).

I guess the art in protocol design is to have as few as possible mandatory-to-implement parts, which are itself minimized in complexity, so that a minimal implementation is doable with a reasonable amount of effort while already achieving a good result (and UX). Then the optional parts can be added piece-by-piece, after the implementation was already published/released.