You don't have to be shredded to be on roids. I believe Johnny was juicing with something, as his rapid slide happened immediately with USADA entering the scene, but of course it's simple speculation without test failures.
If Hendricks was on anything, my best guess would be EPO or something similar, as his cardio and power really just fell off a cliff overnight. Went from being able to go all 5 against GSP and Lawler twice, then couldn't manage to reach the third round without huffing, couldn't throw more than a few power shots a round. I don't think he even wobbled anyone with punches after the Lawler fights? I don't think it was only him aging out of his prime, he was only 33 when he fell off, but I agree his lack of discipline was the primary factor in his overall downfall, juicing or not.
IVs being banned killed his cuts which didn't help. Matt Brown was his last fight before USADA, then he fell off a steep cliff going 1-5, winning only against the ghost of Hector Lombard in the midst of his own slide, getting knocked out left and right, and Hendricks barely took the fight on the cards 29-28.
Johnny had a great start to his career and a terrible ending, all of his own accord. His corner telling him to coast round 5 vs GSP will go down as some of the worst corner advice ever given to a fighter, deservedly costing him a win against the GOAT.
IMO, the moniker of "scariest WW fighter of all time" was RePrime Lawler. Guy is screaming his own face literally off at you, what more are you supposed to do without a tire iron?