At the time, Taylor was running the best system possible for a running quarterback with limited passing skills.
In fact, Roman's system that he installed in Buffalo may be the only NFL offensive system that could result in adequate production with a passing-challenged quarterback like Taylor.
This season has been a bit of a perfect storm: the Shanahan system that is the foundation of what Dennison does requires an NFL quarterback that can pass the ball well. That combined with an offensive line taking time acclimating to a new blocking scheme, a slight downgrade at receiver, and the development of Taylor as a pocket passer (no read-option) has resulted in an offense that has fallen from #10 in DVOA efficiency in 2016 to a current ranking of 23rd (rushing offense from #1 to #28).