Personally (and I think many others do) consider "Availability" as being uptime * bytes, not uptime. Any true distributed system is going to have partial availability all the time (small ranges of data which cannot be fetched within a reasonable latency limit, or somebody's ISP being down). Google accepts tiny ranges of data unavailibility for limited times to not be a real service outage, while the service being hard down for 25+% of users to be a service outage.

It doesn't even really make sense to compare. And since we're dealing with cloud services where the large players have billions of users increasingly providing these services to everybody from small shops (I work for a startup and we use gSuite) to enterprises (many large companies host all their email on one of 2-3 major email providers).

Nothing I said above has anything to do with user experience as you describe it- that's a non sequitur.

It absolutely makes sense to compare. Because the comparison is generally made in the context of using Google services or hosting your own or going with a much smaller provider. One could easily host their own email or other similar service for a very small org and likely have better uptime.