Moral obligation or legal obligation? Legally they would be bound by whatever terms you and they agreed upon such that they are enforceable. Morally? I dunno, that's trickier, but at inception, one surely can't account for all possible future scenarios (acquisition being one) and build that into people's expectations.

I think it's just the nature of business. Even with the best of intentions, a company (site whatever) may not be able to survive (and most people accept that). In the case of failure a mature business can go through bankruptcy proceedings. Still, in most cases, customers are probably the last ones receiving any comp, if any is left. Mostly it goes to creditors and so on.

I think the linked article answers that question well: the ONLY obligation is to be honest, not dissemble about the truth.

Thou shalt not break working code. This is negligence of an engineer, though it's probably imposed by management (and thus not quite his fault) rather than an accident.