A third party code escrow account with instructions on how to build the service and a contract that specifies conditions when a customer gets access to that escrow.
I've been on both the customer and service end of such agreements.
Is there a popular provider for doing this? It sounds like a lot of work to find a solution in which both parties trust (both technically versed and reliability wise). A common notary probably wouldn't cut it, right?
There's a bunch of companies doing it, some dedicated, some as part of larger related offerings. E.g. I know NCC Group (large IT security company) does offer it, TÜV (which does all kinds of certification and compliance work), ...
A guarantee is only as good as the guarantor. A guarantor can be a company providing a service, as well as all sorts of assorted guarantees about said service.
In time, the situation can change. The product can get sold. Cash can be spent. The guarantor can no longer make good on its guarantees.
You're back to square one where the guarantees are as worthless as the original service.
You need 3rd party backing (insurance) in such situations, but that costs money. This money is a cost which makes competing against unbacked entities tougher.
In most cases, you can not have 100% foolproof guarantees of anything. The closest I can think of is governments standing behind their banking institutions. Even there though, governments have defaulted on their guarantees.
The world is not a stable, perfect, and cut-and-dry place as many would like to believe. It is dynamic, and ultimately backed by trust.
It's much harder to mitigate this kind of risk in the world of services as against purchased software.
The problem has always been present especially when larger slower moving companies buy from smaller, riskier, companies.
In the days of software, code escrow was possible to mitigate some of this kind of risk. That's still got it's costs but can be an effective hedge against a supplier going bust.
I've been on both the customer and service end of such agreements.