Yeah, rockium chomp is good but I would guess that dynamax is more of parallel to mega than z move and the only mega that really runs rocks is like mega ttar sometimes.
Also chomp can rock through out the entire game where as drednaw gets a 3 turn span to get them up, this means even if dred beats defoggers it can never put them back up later on into the match so you will just wait to defog later.
This kind of makes dred a suicide lead-esque pokemon in terms of that it only does it once, and I doubt it's very good un-dynamaxed anyways.
Furthermore rockium chomp threatens to wall break basically anything giving it more value than just getting them up which we will only know if dred can.
Also just a shitty side note but, stealth rock probably just becomes max guard when dynad, so other dynas/gigas can not get rocks up