My thinking is that I want experts working on the low-level configurations (power, racks, UPSes, etc). I don't have that expertise myself. Now, I could try to hire some experts to set up highly available hardware from me... or... I could rent some that has already been set up. I think the basic assumption you have to buy into is that your cloud provider knows how to do this better than I do.

I think if we look at a AWS reliability, the interesting and sorta-unanswerable question is: would these applications have been more, or less, reliable, at the same cost, if they were running the infrastructure themselves? My guess: less. It's so easy to screw this stuff up.