Carrick was excellent at retaining possession and he'd shield the defense pretty well but he'd also go missing when it really mattered and he wasn't good at winning the ball back. Excellent at interceptions, mediocre at tackling. He was a playmaker playing as a DM rather than a true defensive midfielder. Fletcher was more of a DM but he only really showed 2 seasons of quality before illness got him. The others you mentioned, Matic/Fred have not done well imo, Freds had what? 4/5 months of semi decent form and Matic had a decent initial honeymoon period before he got pretty shit for us. Legs were gone. Cleverley wasn't a DM either.
We've basically not had an adequate DM since Carrick legs went back in 2016.
Carrick was good in both ways. Although not the most mobile and needed players that could help cover ground around him.
Fletcher, young Anderson, Herrera etc could do that.
I think they have done well. Depend on what you mean with well. We haven't won any titles with them, but I don't think they have individually been the problem.
Poor management and too little quality in attack has been the problem for us.
Matic is getting old, but still class. Although need players around him with more attacking qualities. Pogba have done that, but then did fall out with Mourinho and so on.
During the honeymoon of Ole it worked well even if Matic didn't look to be in peak physical form.
Fred is doing a great defensive job. A bit limited with the ball, but coaching could improve that. Lots of raw talent in him.
I don't think we are going to find many better defensive/holding midfielders than those we got. Some get overhyped like crazy, but do not really do that much.
I don't think Kante is that amazing to be honest because he is limited going forward too. I know people like to talk up their midfield, but they have not impressed me much this season.
The Pool midfielders are great, but I think part of that is Klopps coaching. I don't think they would lift us insanly much even though they are all very good players.
Rodri is pretty limited in my view. Not at Fernandinho level and Fernandinho has got old like Matic.
Ndidi is fine, but not with enough unique qualities really.
Casemiro is very good, but got lots of quality around him that hides his limited creativity.
Busquets way past it.
Pjanic a bit overrated and have not been great recently.
Bayern play Kimmich there since they got poor options for it. (he is good though, but better as right back in my view)
PSG got some good ones, but in CL they still fall apart so hard to know how they are against harder teams.
The only defensive/holding midfield package I would change ours for is Pool and possibly PSG.