Someone reported (it was either here or on reddit) that there are two buses per line. With four parks, there are six lines. Times two buses... that's 12 buses for Express.
Since there are
390 buses in their fleet, that means 3% of their buses have been diverted to Express. Well, a little less than 3% since the Express isn't operating before the parks open or shortly after they close, in which case, they could be diverted to the crowds leaving the parks going back to their resorts.
Speaking of their fleet:
2012: 319
Dec 2016: 390
Plus, their recent order of 50 isn't fulfilled fully yet, there are 10 more coming to bring it up to 400.
This means that the 12 buses diverted to Express are basically new purchases and wouldn't have had any effect on the current bus situation. Whether one thinks the busing situation was good or bad before Express... Express put no dent in it.