I think this is a stupid comment. Two years ago they had Benatia who was the best CB in the league, and they had Strootman who is one of the very best midfielders in the league. Totti being younger is a factor, but he certainly wasn't the "key" to their success.
I believe if right now they had a fit Strootman, and if Castan hadn't have had his issues they would be a significantly better side.
I also think the fact that they've got a defence consisting of a Frenchmen, a Greek, a German, an Italian, and a Polish keeper who mostly have never played together before this season has, and is causing issues. At least with our new defense they all speak Italian, Spanish, or Portuguese.