Not included is overhead of dealing with maintenance. S3/R2 generally don’t require OPS type dedicated to care and feeding. This type of setup will likely require someone to spend 5 hours a week dealing with it.
The majority of server wrangling work I spent dealing with OS updates and, most annoyingly, OpenStack. But that's something you can't escape even if you run your stuff in the cloud...
$LastJob we ran a ton of Azure Web App Containers, alot of OS work no longer existed so it's possible with Cloud to remove alot of OS toil.
b) The seem tolerant of failures so it's not going to be anything like 5hrs/week of physical maintenance. It will be bursty though (eg. box died, time to replace it...) but assuming they have spares of everything sitting around / already racked it shouldn't be a big deal.