I'm not saying he couldn't play well in 2020 and even go on in 2021 but he's 31, his stats took another small trend down in marks/disposals, he missed time with a hammy and he relies on his athleticism to play well. Key defenders who go on to their mid 30's are usually more guys with really good skills and smarts.Disagree on Frawley, i think he had probably his best and most consistent season in his time at the Hawks. Still has a few of his classic Frawley moments with ball in hand but he has been really consistently solid for us. Obviously ageing but not showing any signs of slowing down yet.
I imagine the plan is to give Frost a year to learn the system then make the handover, but Hartley has extra height and skill so is really a better full back option if he could play consistently and then Frost is really good on smalls/medium marking players so might replace Stratton. Either way mature key defensive options are required if you think you're somewhat in a flag window.
Either way I'd be far more surprised if Hartley was playing ruck/forward than if he was a useful defender.