While I agree in principle, in practice the result would probably be terrible.

Just look at tables in GFM, which mostly were designed as you propose. They look like tables even before rendering, however the experience of actually creating and editing the tables is not fun at all.

I have a horse in this race (co-founder of a markdown-based note-taking app) and to me, the benefit of markdown is easy + explicit editing, not that it looks the same in plain text vs rendered.