I agree but that's beside the concern I'm raising. How do you ensure freedom on the non-free part? The answer is in the question, I'm afraid.
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 :-))