The main goal of the brand split was to have two rival brands that would eventually develop new talent, and once the next boom period occurred, the WWE would benefit from the respective PPV revenues and house show gates of each brand.
That is the intent of the split, but WWE hasn't acted accordingly to support its own intent.
With Lesner leaving the WWE, McMahon pressed the panic button and held a draft to hopefully 'level the playing field'. However, in hindsight, it didn't work out as planned.
Booker T, who had been buried by HHH, and was a mere upper mid card (at best) went to Smackdown and alleged it was the minor leagues. <~~~THIS IS WHAT LED TO THE DOWNFALL...By Booker T acknowledging Smackdown as an inferior brand, what reason did fans have to watch the show? And when Booker made that comment, Angle (GM at the time) did nothing to disprove or refute Booker's claim! In the back of fans mind, this illustrates that Booker's words rung true, thus why should fans have to watch an 'inferior' brand?
IMO, booking is to blame-
RVD receives some of the biggest pops, despite being continuously buried. In a tenuous depth roster, many believed RVD would flourish. WRONG! RVD is now the featured attraction of the Smackdown B show.
How did this come to be? Bad booking, but more importantly, the WWE is holding down RVD. They should just let RVD wrestle his own style, the same style that made in such a fan favorite in ECW. However, I won't think that will ever come to fruition.
There are more booking mishaps, but I'm sure you are already familiar with them.