Thats not how it works. Most of the time even our double pivot with Fred and McTominay works as a midfield 3 with Bruno the furthest forward midfielder. Turning to a single pivot just means to have one of the double pivots pushing on more and letting the lone DM along with the CB's to cover for any counters. That won't change unless we bring along another midfielder who can do that role or develop one of McTominay or Fred to do that. Matic just doesn't have the legs to do it. In fact making it a single pivot as opposed to a double pivot means Matic has to cover more space.I checked that on fbref, he played 27 games at DM position in 17/18 and got 2,37pts per game in those games. In total we had an average of 2,13pts/game that season so that's actually quite good record from him. We played Matic as DM with Pogba, Herrera or Fellaini.
I did not include games where he was DM/CM/CB because I don't know what happened there.
He literally has two CMs to support. Are you suggesting it's actually more difficult to play midfield three than midfield two? I don't get that.