Except it's not that simple. iPads run on ARM processors. Macs run on Intel processors. Even the maligned Core M is faster at most tasks than the A9X. However, the Core M isn't as efficient as the A9X, and would need to run iOS apps in emulation, which would hurt both performance and battery life. Similarly, the A9X isn't powerful enough to run Intel apps in emulation. It would have 2007-class performance at best (and people complained about the 12" MacBook having "2011" performance).
More likely, Apple is going to add features to the next versions of iOS for the iPad Pro to make it more capable of the remaining tasks where people still use Macs or Windows PCs.