Having something you can sell profitably makes it more likely that the open source part will stay open.
The thing you sell also don't need to be proprietary. I work for an open source company that sells free software [1], support, hosting and consultancy. It specifically rejects doing open core in our business decisions. I'm glad it has worked so far (and has been for 20 years this year).
(coincidentally, CryptPad is one thing the company makes :-))