In particular you cannot provide an internal SLA on write requests to low-level storage solutions if you can't guarantee that you actually have the spare disks.

The SLA would have to be something like "we guarantee 99.999% write availability unless team A or team B or team C makes a mistake", so now you have to look up what team A and B and C are up to and what do they guarantee. Instead of the more sensible "we guarantee 99.999% write availability unless you are out of quota".