The Rock is simply pandering I don't know why the superman fans don't see that. Black Adam and Superman have no rivalry let alone animus towards each other to warrant him trying to hype up a matchup between them. He should have been campaigning to fight Shazam but that's his problem he views fiction as one big wrestling booking gig and not actually caring about the characters.
Black Adam and the JSA failed
Shazam is on its own in a sequel will also fail
The smart move would have been to include Black Adam in the shazam movie and then(based on the reception to that) spin off into the origin story/post shazam 2 plot with the jsa
Instead they spent maybe 600 million on two movies that will not make them a profit
These people in charge do not plan anything out and the rock and other high profile celebrities can campaign for a big paycheck through their vision instead of the other way around. Affleck did something similar with his Batman movie trying to set up a deathstroke conflict when imo you already have joker, harley quinn, and several other of his rogues gallery already cast.