Escrow
We, a small software dev group (a tiny part of a big company) write software that is vital to at least one much larger company's operations.
On at least one occasion we have been requited for a project to put our source code in escrow. So that if we disappear the big customer will at least get the source so they can continue to (try to) maintain it for their own needs.
So closed source does have ways where a customer, if large, can take steps to ensure its continuity.
Biting the hand that feeds IT
