Yes, he caught it, secured it, and possessed it...but then he didn't. You don't seem to understand the concept of "completing the catch". He didn't catch it on the run. He caught it in the air, and stumbled forward to the ground. That means he has to keep possession the entire time. When he hit the ground, the ball hit the ground and popped loose. I repeat, the ball HIT THE GROUND, and POPPED LOOSE. That's not maintaining possession. But if you've been watching football close enough the past few years, you already knew that, and you're arguing just to argue.
But him turning around, switching hands to secure it, taking steps, negates that. Calvin Johnson was in the air, all one continuous motion. Whether he lost his balance or dove, is the argument, but possession and steps were established prior to. I think he was going down, so tried to extend to goal line, with the secure hand. That's a catch, since he clearly took steps with the ball in possession. The challenge, if any, should have been down or fumble TD.
No one is arguing him having possession of the ball, or him taking steps, or even trying to extend to the goal line. So all of that is seemingly, unanimously agreed upon. The argument is him losing possession at the end of the play, and how is that argued as not having control throughout the end of the play..when everything NOT being argued is saying that he already had possession.
But in the end, the fix was in to unfix the prior fix.