Given George Lucas' referral of Anakin Skywalker being the "Chosen One", I think GL wanted the Windu vs. Palpatine match to be an even one where the deciding factor would be Anakin.
If Anakin chooses to resist temptation and respect his elder's advice and wisdom in stating Palpatine must be destroyed then he helps Windu. Ultimately, it was his son Luke that had to reject Palpatine in the final movie thereby prompting Anakin to do his duty as the "Chosen One" and destry the Sith by killing Palpatine and himself.
But, Anakin decided to take the dark route and help Palpatine thereby aiding and abetting the killing Windu. Followed by committing serial murders in the Jedi Temple, and in the Separtist HQ at Mustafar. And, Anakin was just getting started . . .
I think the Windu vs. Palpy battle was also the start of the Sith gaining the confidence that they were destined to rule the galaxy. They beat Windu and Yoda in the same week in order to gain control of the galaxy while the Obiwan/Anakin battle was a tactical loss, but a strategic victory. 25 years later, there is little wonder that Palpy was pretty smug when he saw Luke try to challenge him in the 6th movie. I mean how could Luke compare to the great Jedi they beat to destroy the Old Republic in the first place? But, then betrayal is the way of the Sith: Palpy betrays Dooku by having Anakin kill him, Palpy betrays Anakin when Luke beat Vader by goading the son to kill the father; thereby, inspiring Anakin to betray Palpy by destroying him.
You could say that the Sith had a capacity for self-destruction which challenges humanity everyday (ie. also a common Battlestar Galactica theme that Cylons have about humans). Or, as Wellington put it during the Napoleonic Wars:
"The only thing I am afraid of is fear. "
"Wise people learn when they can; fools learn when they must."