What good reason did he have??

He did not know what would happen over the next month. He thought he had already won (the battle and the campaign), and that there was no reason to commit the guard. Contrary to his subordinates, as you say.
Would a more decisive victory at Borodino have changed anything? Would it have compelled Alexander to capitulate?
We don't know. But we do know that the Russian army was able to retreat in reasonably good order.

The Russian army wasn't broken or destroyed, which was his objective since day 1. He finally had his chance, as late as it came, and he didn't take the decisive course of action.