I think Thornton likely does have more success if he had, say, Malkin or Jagr to split defenses with.
Same goes for lots of players who, through no fault of their own, didn't have much in the way of a running mate (Marcel Dionne, Jarome Iginla, etc).
I am not sure if this is still a knock on Mario, but there are few players in NHL history who could score with plugs on their line or stars. Mario could do both. Check out the career of Wendell Young. You might notice a singular season that sticks out. That was rookie Mario he was playing with. Rob Brown comes to mind. When you are simply that good as Mario was then you are the reason for your success, not another Hall of Famer you play with.
Ovechkin may well end up as the only Top20 player in history not to have another Hall of Fame teammate over the course of his entire career (except pre-retirement Fedorov). Backstrom, Green, Oshie, and Holtby are not making it.
Backstrom I would say has a decent shot at it. Might fall short, but if Getzlaf is getting in then a contemporary like Backstrom isn't outlandish either. Either way, Green, Holtby, Backstrom, Carlson and even Semin all at least had seasons or peaks where they played like Hall of Famers or that they could go in that direction. Barry Trotz as the coach when they won, that's as close to a coach being in the Hall as you'll find. Throw in Kuznetsov too. Other guys like Oshie, Justin Williams, etc. lots of support Ovechkin had on those teams. Those Caps were very good teams, they led the NHL in points three times in Ovie's career.