Maybe the number of people who would use this feature is no enormous, but I still think there is a gap in the market that Proton could fill. Importantly, the people who would use this feature is willing to pay.

There is essentially one popular option in the market which is Overleaf, which requires a paid subscription to share a document with more than 2 people, and it is amazingly expensive (€19 monthly). Yet many people pay for it (research funds, it is the university who actually pays).

Probably this is not immediate to build, but maybe in the future...