I think it is, and I'm losing interest myself. I've been running dynasty leagues since '97 (16 years) and I've recently thought about the reasons why my interest in fantasy football is waning (dynasty leagues, especially):
1) The death of the bell cow RB. It used to be that you could obsess about the draft, sell your soul to trade up, pick a RB, and live or die with him for many years on a fantasy roster. Now it seems players (especially RBs) are rotating through careers or as part of a RB committee so frequently that the sense of ownership/accomplishment you got from drafting them just isn't there anymore. Now we have 3rd down RBs. We have goal-line RBs. We have situational RBs...
If all NFL teams did this, then it would even out. But as we know, there are still some NFL teams that have that bell cow RB who never has to share, like your Adrian Peterson, and having guys like that on a fantasy team is such an incredible advantage while other teams are forced to fill 2-3 roster spots just to add up to one "starting" RB. It really has become an unbalanced affair, which brings me to my next point...
2) ...rules for most fantasy leagues rarely remain flexible enough to adapt to a changing NFL. I don't know about you, but I've found it near impossible to get even the smallest rules tweaked within the various leagues I'm in. You're generally required to have a high percentage of owner support for rules modification, but I've always found there to be 2-4 owners who refuse to change. At all. Ever. No matter how silly not changing is.
Eventually, you just get tired of dealing with them.
3) Fantasy football is not interactive enough to the new generation of fans. Pretty controversial opinion here, but there is scientific data to support there being fundamental differences in thoughts/interests/personalities between the various generations (Baby boomers vs Gen X'ers, vs Gen Y, etc). The current generation of younger football fan (teens/early twenties) is used to flashy Xbox, immediate feedback reinforcement and constant gadget interruption that staring at a list of players and stats and a draft board for something that will reward over the course of several months just doesn't "do it" for them.
I also don't think the NFL does enough to promote fantasy football to the younger audience, but that's a topic for another day.
4) The invention of the DVR. I used to watch football live and keep track of my players between commercials. However after using a DVR for several years, I've found that I just can't watch commercials anymore and there are few things on TV with more ads than an NFL game. Now, I generally record the first hour of a game, then start watching so I can fast forward through the gauntlet of ads. Because of this, I now don't have the added excitement of realtime player stats, as the stats I'd be getting would not match up to my current football watching timeline. So... checking player performance has now become something I do toward the end of the afternoon. Therefore, the excitement of game day has really been taken out of the picture.
5) I'm getting older, and my priorities have shifted. Self-explanatory.