Top 5? Maybe, but not EASILY. He might not be one of the top 2 backs in his division. When the O-line was healthy, David Johnson looked like complete garbage.
Gurley, Barkley, Kamara, Gordon, McCaffery, Elliot. Which two guys are you kicking out to make room for DJ?
Because Josh Rosen was a rookie playing the most difficult position in sports and David Johnson had just signed a contract making him the second-highest paid player at his position.
David Johnson finished 42nd in the NFL in rushing DYAR, 40th in DVOA, and 43rd in success rate. He had the third-most runs in the NFL.
Johsnon was even less efficient as a receiver. 50th in receiving DYAR and 43ed in DVOA. He had the 11th-most targets among RBs and the forth-lowest catch rate (66%).
I don't think that DJ is THIS bad, and I expect significant regression upward next season; he's going to be THE value pick in fantasy next year. But this doesn't change the fact that it was ludicrous to extend him at the beginning of this season without knowing how much of his success was dependent on context.