You can try to argue all you want, but the stats are really convincing. Every game Trubisky played in, both before and after his benching, aside from the Saints playoff game the offense ran for over 100 yards. Every game Foles played in, the offense ran under 100 yards.
Yes, the offense was working through things, but it simply could not find any sort of success under Foles. Defenses stacked the box against him, covered the short to intermediate plays, and were not threatened at all by the deep pass or Foles ability to escape the pocket.
There's no way to slice it last season except that the offense was far better under Trubisky last season than it was Foles. If you want to hold the play of Watson, Mahomes, and a very bad 2019 year against Trubisky, then go ahead. But when simply looking at the recent play of last season, Trubisky he played serviceably. If you have a better value option at QB on the market, let me know.