Blizzard's intention is to provide a systematic grouped styled gameplay that is fun while allowing people to grow in character power. Blizzard's idea of that is through raiding/dungeon as they believe that is what players want. For players who enjoys raiding, content is there. For players that only want higher ilvl, the carrot is there. For both groups, content lasts long enough to entice ppl to stay subscribed until Blizzard is able to push out next batch of content. For people who don't enjoy that then they will unsub like they do with any other MMO.
So far, there are only a handful of successful MMO that doesn't use raiding as end game that I am aware of.
EVE online : crafting and pvp and making money through trading
Some chinese MMO: no level cap and unlimited power growth through level grind
Most of the other MMOs are play till cap and maybe play for a few more months then quit. Maybe come back and resub when new content comes out but they don't stay subscribed (sounds like WoW subscribers that don't enjoy raiding).
Anyways, Blizzard's hope is to get ppl to raid as that can get subscribers to stay subscribed. They have determined that it is not possible to create high quality content faster than players can consume without gating in some way. Only allowing players to kill a particular raid boss once a week hasn't caused QQ in the western world so that is staying. This is largely frowned upon in asia so raid lock out works differently over there as a result.
It all comes down to how to get people to shell out 15 a month on a regular basis. If you have some awesome idea on activities that are fun and will get players hooked for months (or years) while paying a reoccuring fee, definitely submit that suggestion on forum as I am sure Blizzard would love to hear that idea.