Time sinks aren't challenge. Getting mythology in this game isn't a challenge, it just takes time.. but it's certainly not difficult. Time itself doesn't automatically mean something is a challenge. A boss fight can be extremely challenging, and take less than an hour to down (when you do it correctly). I'm not saying challenging things don't take time, but just because it took longer to do something does not automatically mean it was difficult. People seem to confuse the two. I also don't think everything in a game should be time consuming or challenging (unless we're talking about appealing to a niche audience, which most games don't aim for).
I'm an adult, my first MMO was Shadowbane. I played a bit of Ultima Online, and Lineage as well. I know the grindy stuff that people seem to want, and I can harken back to it myself with a bunch of nostalgia. However, when I sit and think about why I enjoyed something, it often has very little to do with how much time it took to do it. Raiding (whether open world or instanced) takes a long time, but it's not an accomplishment because of that alone. It's an accomplishment when you finally dodge all the attacks, coordinate well enough, and down the boss. The time itself isn't really what's causing the feeling of accomplishment. I'm not actually locked into any time constraints. None at all. However, I recognize that with the MMO industry being as large as it is now, a good majority of the people probably don't have 5-6 hours to devote to an MMO just to make progress anymore, and I can accept that as an okay thing. I like more people in the industry, it's going to mean more innovations. I don't want to restrict it to a certain group.
Challenge is good, I welcome it. I have raided in any recent MMOs because I like the challenge of doing so. However, I am not going to confuse killing 1 million mobs to get an item for a quest with actual challenge. Just like spamming AK/WP isn't a challenge (and wouldn't be, even if you made mythology take a year to get), killing little things for hours on end is not a challenge.. because time does not equal challenge.