you can find examples all season of guys catching a ball, falling down and fumbling.You have to survive the ground cmon. My example above is the most plain way to explain it. You're telling me, that if he was all alone no one around and he seemingly catches it with 2 steps hits his knee and back and the ball pops out that's a completion? It never is. To continue that, the ball also during that sequence never hits the ground and is in the possession of the defender on the conclusion of the play
One of the most famous TD in SB history by Richards was ruled a catch that would never be a catch today.
You can also find examples of catches being made guys sliding on the ground and the D ending up with the ball and it being ruled a completed catch and the player being ruled down.
This was not 2 guys catching the ball at the same time and one ending up with it