I just think that the whole downfall of the Jedi's lies with Anakin, and his turn to the dark side. If Anakin didn't turn, none of this would have happened. Windu would have killed palps, and order 66 would never have been given, and the empire would have been crushed before it even started.
So starting with the premise that Anakin is the cause of the Jedi's downfall, then where do we go from here?
Of course there are many things that led to his ultimate demise, and i'm sure all of them weigh in there somewhere. I'm just saying that like anything, we go to the biggest cause first. There are dozens of things we could say that helped, but his poor training was number 1.
Now don't forget, Windu says "No, he will not be trained... he's too old" and Yoda also says "the chosen one he may be, but grave danger I fear in his training"
Obi wan says " I will train the boy, without the councils permission if necessary"
Yoda again "Qui gons defiance I sense in you"...
Obi wan insists to train him and they VERY reluctantly agreed. So not only did Obi wan train someone that the council said would not be trained, but he didn't do a very good job of helping Anakin work through his feelings for his mother which sparked his problems to begin with.
It isn't all Obi wans fault of course. Many things could have happened, but I think he is at least the most to blame.