But that's the point of contracts, right? When a company shuts down, the contracts become part of the liabilities. E.g., if the contract says "you must pay each customer $1000 if we shut down" then the customers become creditors in a bankruptcy proceeding. It doesn't guarantee that they get all (or any) money, but their interests are negotiated by the bankruptcy judge.

Similarly, I can imagine a contract that says, "if the company shuts down, all our software becomes open source." Again, this would be managed by a bankruptcy judge who would mandate a release instead of allowing the creditors to gain the IP.

Another possibility is for the company to create a legal trust that is funded to keep the servers running (at a minimal level) for some specified amount of time.