What's needed to form a contract is an offer, acceptance, and consideration - if an offer was made and accepted and something of value was exchanged, and there wasn't confusion about the terms (after 10 years, there wouldn't be), that's good enough for a contract to have legal force.

Not forever, no. But usually with any sort of contract, when one side wants to break or alter the contract they have to give the other side adequate notice. In certain circumstances a contract can eliminate such a requirement, and the definition of “adequate” would vary a lot based on specifics, but that is very much a possibility in this case.