Building a rich text editor with Portable Text may require writing from scratch, but Sanity's editor could be used outside of the studio.

So if I understand correctly, Portable Text was published as an open standard, with some open source tools to convert Portable Text to other formats. But if I wanted build a rich text editor that spits out Portable Text, I have to write that from scratch, correct?

Per today, yes, probably.
Here’s a crude proof of concept
https://codesandbox.io/s/portable-text-on-slate-proof-of-concept-l6xmo
That being said. We have just refactored Sanity’s rich text editor, and if I'm of the correct impression, it's not that far from being able to be used outside of the studio as well. It’s at least an ambition that we have.

Per today, yes, probably.
Here’s a crude proof of concept
https://codesandbox.io/s/portable-text-on-slate-proof-of-concept-l6xmo
That being said. We have just refactored Sanity’s rich text editor, and if I'm of the correct impression, it's not that far from being able to be used outside of the studio as well. It’s at least an ambition that we have.