TBH, I always thought Johnson's TD looked incomplete (unlike Dez's catch)
It is all about the rule.
If the rule says you control the ball and get two feet or one other body part down than it is a catch the instant the second foot hits. Anything after that is irrelevant if it happens in the endzone. There is no time or football move requirement.
The Beckham catch was a TD the instant his second foot hit since he had control of the ball. The defender knocks the ball away a split second too late. The foot hits its a TD since they were in the endzone. If they were in the field of play that would be a fumble.
Butch Johnson dove for the ball, caught it in mid air and landed shoulder first in the end zone. The instant his shoulder hit in the end zone it was a TD. If he hit at the one it would be a fumble and a live ball.
Golden Tate was an interception since the defender knocked the ball loose before Tate got his second foot down. If the other defender dropped the ball it would be incomplete.
Dez was obviously a catch since he took 3 steps with control before he reached out for the goal line. Since he was contacted on the way down he down by contact.
Two feet and control and its a catch anywhere on the field whether you dove, jumped, stood still or ran.
If you dive and the ball comes free before a body part hits then it is incomplete. If you dive and get a knee or elbow down before the ball comes free it is a catch. If you were contacted it is down by contact, if not it is a fumble and a live ball.