The artificial distinction between a prototype and a production system rarely survives reality in my experience. But that doesn't matter because the situation w

The artificial distinction between a prototype and a production system rarely survives reality in my experience. But that doesn't matter because the situation with long-term maintenance is not so different with freelancers doing the development to what you'd have to do with an in-house team. It's rare for anyone to be around to look after the code they write for you forever. Ironically you might have more chance of a good long-term relationship with some of your freelancers - having a few good clients who come back to you from time to time is an excellent strategy as a freelancer. So you always need the system and processes to be designed with future maintainability by other people in mind. Good developers will do that whether they're employed or freelance. Bad developers will mess it up either way too. Choose the people you work with wisely and try to make sure when you do bring in new people they have a good way to get up to speed so they can naturally take over as others leave. There's not much else you can really do.