Why is everyone so dead set that Sasuke couldn't have improved enough to beat Itachi? He's not exactly a slouch and he's certainly had plenty of incentive to improve himself over the years. Oro even thought Sasuke even better than he was at the same age, and that was before Sasuke absorbed Oro's abilities into himself.

Now, we don't know if Itachi's had more incentive to strengthen himself since joining Akatsuki or not, and we don't even know what the limits of his abilities were before the time skip, but if he hasn't improved that much in the time between leaving the village and now then I would think it's at least possible that Sasuke caught up to him considering he's been trying to improve himself constantly for all those years.