Guaranteed availability is a bet they're willing to make, a gamble they've been on top of so far, a risk that, should something fail, they will pay out on according to their SLA.
Seems to be working out for them (Ably) and this isn't a public billion dollar company like Dropbox.
I would have thought that given Dropbox's engineering talent, they would have designed a system that would account for 5x9s and even making that guarantee for enterprise or mission critical customers.
Can't even find their SLAs anywhere for these customers, so I presume that Dropbox doesn't care about them.
Guess I was wrong and this is just disappointing and made the move not worth it.
https://aws.amazon.com/blogs/publicsector/achieving-five-nin...
Here is a service that has managed to achieve 5x9s of availability:
https://ably.com/