I agree, there was a lot more than just Brady regressing last season. However, the film is still telling. He didn't have to overthrow open receivers, didn't have to throw the ball at the running back's feet in simple screens, etc. Brady has been regressing for a while now. These throws started to appear in early 2018, first sporadically, then more frequently. In 2019, it was way worse than before, and gotten worse as the season went by. Who knows how it continues in 2020? Maybe the gradual regression will continue, but he'll still be able to compete at a top10-15 ish level, maybe it will accelerate.
So the Bucs took a risk, no doubt. But the Colts took a risk too. That is what it is if you don't have a franchise QB. You have to take risks.