I do not see how this helps. I'm not trying to make AD FS robust against server downtime. I'm trying to cope with Internet connection downtime. If I lose connectivity to a site altogether, because Comcast is having an outage or our building in the UK is doing some power work (both things that have happened recently), it doesn't matter how many servers I have at that site; people are going to lose access to their e-mail.
I don't understand how Active Directory Sites and Services is supposed to help me here. It's Office 365's access to my public-facing AD FS endpoint that matters, not my internal DNS.
I am seriously considering password sync rather than AD FS if the latter can't have site redundancy.