The Flynn at R2 discussion is entirely about contingency.
If Flynn plays 8 games and averages anything over 60, you've won and every point over that is just gravy. Even if he scores in the 50s, you're fine. He really just needs to be playing and not scoring 30s consistently!
The problem is if Flynn doesn't play for whatever reason. If there are other rookies who look set to play, then they could cover. McEvoy is the obvious other cover choice to me but the problem with him is you have to lock in Biggie from the first game and couldn't take another rookie ruck if they were named or you need to trade him in when Flynn misses that could derail upgrade plans/cost you a cow and effectively undermine all the value. It's a shame that Biggie is the only D/R option. You might end up losing another valuable cash cow for cover you don't even need, although everyone is probably missing him at least.
The difference with Flynn to any other rookies (non-rucks) is the pure amount of contingencies. If 123k rookie at other position isn't playing you move him to the bench and get on with things, if it's your R2, you panic or pray.
If Marshall was fit, there would be no discussion here, you'd just lock him in at F2 and Flynn at R2 and hope you don't start the right one of Grawndy!