I think MNB has a good point. Zhang Jiao's agenda consists of "overthrowing Han", but after that, nothing. His skill of organising people was amazing, as it was not simple to get so many people all over the place to rise in rebellion and strike such a blow to the current infrastructure. They even had a few victories over the government troops earlier on.
Though most of the government troops were no more than recruited peasants given swords, and Zhang Jiao's people were fuelled by passion, there was no way the Yellow Turbans could have won. They lacked a political agenda, and did not secure enough support with those with power/money/education. Though they had bribed one of the top eunuchs (I forget who), he turned double-crosser to them and betrayed their uprising date.
Even if they could overcome the government troops, internal struggle will arise, and like others have said, other warlords with more power/money/intelligence will come and crush them.
"Whatever you do, don't fall off the bridge! It'll be a pain to try to get back up again." - Private, DW 8