Only problem with putting the Rock in the Pred costume; he is too short for that. His build is about right, but there are 7 footers who are built like that too.
I agree that a lead actor has to ultimately be a good actor.
There are some well built actors who seem hardened and masculine and would be great for character actors in a military unit.
In Iraq if you were not on patrol or some mission, or not prepping for the next one, or eating etc... You had little better to do than hit the gym. Killed boredom, and the goal setting helped keep you sane. Something to look forward to each day.
There were a lot of hobby bodybuilders in the Military when I was in Iraq, some very jacked dudes too. I am talking 6'4" 250 pounds and ripped. Some even bigger than that. So their place in an action role is fine with me, and does not seem unrealistic, unless they are all built like that.
I was not quite as big as that myself but still awsome shape when I was active, I am 5'8" and was about 215 and lean at my best, but I would go to the gym, walk in and see some old as dirt NCO who is bench pressing 500+ pounds for 12 reps.
If they want a military or ex-military hero, and they want him to be huge and ripped that is fine with me because there are some real life heroes, I have personally met who are jacked. Not unrealistic at all. But they have got to be able to act.
There are some unknown actors, legitimate schooled actors, who are very built, not quite built like Arnold, but there are plenty out there who are close AND can act.
The problem is studios would rather cast a Pro Wrestler in a "muscular guy role" than an actor who bodybuilds as a hobby.
It is not like spending 2 hours in a gym on a regular basis is going to make you stupid, but if you have zero acting schooling, than you will not be credible to the audience.
I do not think your average "joe shmoe" civilian character who rises to be a hero should be powerfully built man. Most folks are not built like that, and other than a military lifestyle, no lifestyle of that than bodybuilder or pro athlete ever comes close to actively encouraging people to be in the gym.