Are we not in danger of overthinking all this?
In the next 2-3 years, Berno, Derno, Flynner, Kev Mc, O'Gara (who are all in the 29/32 age bracket) will have called time on their inter county careers, will be about to, or will have moved to a Dennis Bastick type supporting role off the bench. Add Paddy Andrews to that list, if his run of bad luck with the injuries continues. That will free up 5/6 starting sports in the forwards. It's the natural order of things.
Mannion, Costello, McHugh, CO'C (and probably AN Other - the bolter, yet to be discovered) are the future, whether we like it or not. We're not going to find out just how effective they can be, until they are ones running the show, day in, day out and the Old Guard are up in the stand, showing Kieran Donaghy the AI medal(s) they robbed off him.