My bursar and I were discussing this years ago. It's the sort of thing that occurred to him just because of the different businesses he has to deal with. Occasionally, someone DOES go bankrupt, and the clients are left to pick up the pieces. As soon as any mention of putting necessary services (i.e. not just "run our website", which we can hand off to anyone within a day or two if there's a problem, and even have "backup" domains for, nor "provide online content" - but actually real services like thin-clients, running our library, etc.), then the question quickly arises about what happens if they go bankrupt.
Hell, I get staff who complain when BBC Schools change the content they have on their website, like I'm personally responsible for them phasing out 10-year-old Realplayer streams that the teachers have had in their lesson plans since before I started and not providing a replacement. They honestly expect me to "do something about it" (I don't know what, because all I can do is send a sternly-worded letter to the BBC, same as them - the way they talk you'd think I could just claw back years of archived copyrighted video material without the BBC being involved at all - hell, even Archive.org doesn't mirror RP streams).
So when we put something in the hands of an external business, we have to basically assume that one day they won't be there. It's not a problem - companies come and go - but we just plan what to do in that case, but we *do* have to plan.
My bursar in fact just read this post of yours, and we weren't surprised at all. The correct answers are obviously the last two - escrow accounts and DIY. Like RAID, backups, and everything else, you should never assume that something WON'T go wrong - you should just plan for it going wrong and finding out how you would cope with that. You can't stop things going wrong, but you can stop them being big problems if they do.