I know this doesn't make a whole lot of sense (because frankly, not a lot did in that final fight) but when Yhwach said that "every
Bankai has already been crushed in the future", I assumed that Ichigo somehow shifted his powers back to Shikai form as that was the only way around Yhwach's future-modification at the time. Yhwach may have crushed his Bankai in every possible future, but not his Shikai, which is how he was able to beat him.
But it doesn't really work because it doesn't seem like something Ichigo consciously chose to do, nor does it explain why he used his old version of Shikai.