I started raiding in Rift about the same time Hammerknell came out. Amazing instance. Hardest content I'd ever done for the most part. Took me and the guildies quite a while to get through there. All one difficulty aswel, so when you saw someone geared out in HK armour you knew what they went through to get it. Good times.
If GW2 5 man dungeons are as hard as the developers are making it out to be then I think people will be playing for longer than most people did in WoW/Rift raids. It's not the size of the groups that make it fun or hard. It's the content.
Dynamic events and finely tuned 5 mans might even be like nothing most of us raiders have come accross simply because everyone will be in equal-ish gear. Developers won't have to factor in groups with an extra 50% DPS from gear levels alone when balancing an encounter. The scope of players that the content must be tuned for will be very small compared to what we are used to. No more bosses made for groups in full blues and full epics that have to be hard for both groups. Or bosses that favor this particular healer because they can HoT while CC'd or whatever the case may be. The same can be said for PvP too. Balancing such a small gear-envelope will be much easier than trying to make rogue vs mage somewhat balanced for PvP blues players and 2500+ arena games.
I have a feel that is way off topic....
Can't refute the numbers. Box sales alone can support an MMO for years if it sells well. Factor in the micro/cosmetic transactions and you've got a situation in which you really cannot fail from a $ standpoint.