Yeah, it would have to be somewhat automatic and widespread, sort of like credit card warranty/travel/etc. insurance.
It might make sense for b2b apps instead of consumer. b2b apps still aren't usually sophisticated enough to do source code escrow (and what does that really mean for SaaS apps), but a business would still be hurt if someone like Salesforce disappeared (or even a smaller product like a CRM gmail extension or whatever, if your workflow is built around it.)
Also, developers are customers of sites with APIs, so some kind of promise that an API will remain available, non-throttled, etc. would be good incentive to adopt it.