Erm...
Windu couldn't beat Sidious, but Sidious couldn't beat Windu EITHER. The more darkness Sidious channeled, the more Windu would be able to turn around to use as power to fight him. Had Anakin sided with Windu, Sidious would have failed.
However, after that moment, he'd won strategically. Beating him in a fight wouldn't have defeated him. A different approach was required, because no amount of power could have DEFEATED him - even in death, he would still have won. Vader would have taken over as Emperor, and even if he didn't the Republic would still have been hunting the Jedi. By the time that fight happened, its outcome was no longer relevant - such is the brilliance of his plan.