How often is it offline outside the maintenance windows?

Yeah, maintenance windows are annoying, but they don't really count when describing the availability of a system. Many government systems are only available during normal business hours. That means they're offline for 16 hours per day. What matters is how available they are when they're supposed to be working.

For Elder Scrolls, two nines would mean that the game was allowed to be down for more than an hour a week outside of those maintenance windows. Or, if measured by quarters, which is more typical, the game would still have those maintenance windows, but, in addition, it might be down for a full day once per quarter.

Basically, the 8 hour windows every Monday is a trade-off so that the rest of the week is uninterrupted. They probably manage three nines the rest of the week by shifting any serious maintenance into the weekly downtime.

And, as for the game being "still popular", one site says that there are currently 7199 players in Elder Scrolls but more than 161k in World of Warcraft. It could be that part of the reason that World of Warcraft is more popular is that it doesn't have 8 hour maintenance windows every week, but it does often have 2+ hour windows. The number of players who are willing to put up with 8 hour maintenance windows every week seems pretty small.