Stanton trained with defenders all preseason and did reasonably well in preseason games as a HBF'r. It's irrelevant it's his first year doing so.
At the time he came in for Kelly our main choices would have been Stanton or Jerrett for that role as both had been playing it in VFL also. Given we had both McKenna and McGrath as inexperienced players they went for the experience of Stanton.
Same goes for Howlett who'd been training with the fwds a lot through preseason, and slimmed down accordingly to ensure he could provide the pressure needed of smaller fwds who pinch hit in midfield as required. Green dropped out, in came Howlett (who was BOG in VFL).
Fact is neither are seen as part of our proper midfield mix anymore. Hence they were training preseason in other areas.
Them being selected when they did was not a justification of picking them for sake of it. Selection committee clearly saw them as best filling the role needed by outgoing injured player / most likely to impact helping team win.
Until younger players are demanding being picked ahead of them for specific roles then that will keep happening.