You're making some assumptions. For example if you use epaxos for the quorums there is no unavailability due to a leader failure and re-election. Even then, re-election is likely going to be a lot faster than any sort of fault tolerant 2PC coordinator recovery protocol. You're exaggerating the write amplification. The above offering is single region. Google says they do 5 way geo in their various papers/etc, and there's no way they're paying some cumulative 15 copy write amplification as you imply.

I've worked on a storage system that used 9. 15 doesn't seem impossible to me.

epaxos is not general purpose replication. It leaves the problem of reconciling multiple replication streams to another layer. I've heard that they do 20+WA (distributed, before you consider on-disk WA) for much of it. shrug

9x write amplification isn't that high. If you are setup with a master slave configuration, that creates write amplification and for reliability, you probably have some sort of raid, that increase the number of places writes go to as well.

Not necessarily RAID, it might be an erasure coding.