I feel that owning a code to custom written product is akin to owning a painting.

Sure you paid an artist to paint you.

And, as you get older, uglier and/or get new wife or more kids, you'll have to pay said artist to paint a whole new painting.

Same here with Crestron.

You paid for a solution and you got finished product.

Changing equipment, system updates, etc. will require a change/update.
If existing dealer does the work, he 'should' have the working file.
If you go with new dealer, he should not 'steel' previous dealer's IP (Intellectual Property) methods, graphics, etc.