Right. Says you. One person. Didn't know you spoke for the masses.
Since I think II is the best in the series.
But anyway...
Crysis 3 is the shortest game in the series, but Brotherhood with all of its (meaningful) side content is just as long as any other of AC series.
Brotherhood expands on all of the mechanisms of 2: exploration, dungeons, combat (huge improvement), and resource investment. It also adds Assassin Recruits. The only thing that was a regression was location variety. On the other hand, Crysis 3 gets its length chopped in half at the expense of the bow - otherwise, 2 & 3 are more or less the same game.
You argument is precedented on the fact that Brotherhood, like Crysis 3, is nothing more than an expansion to the previous numbered title in the series. This is objectively wrong; Brotherhood has just as much content as 2, if not more - it's everything a sequel should be. It's my opinion that Brotherhood is the best (but seriously, it's a pretty wide-held opinion here on GAF), but you're wrong in calling it a write-off.