Sure Matis is a better 6 than Herrera, but the poster we saying he could have played as 6 behind Bruno and Pogba and I agree, he would have done a good job there for several matches, even if not as a permanent starter.
Even when Fred improved in the first half of the season, he definitely didn't do anything much better than Herrera did for United. Herrera's peak form is still much better than Fred's peak at United so far, and as for the poor start, I won't go on to compare Herrera's first season to Fred's start with United because we all know about it.
Point is Fred didn't replace Herrera. Herrera was a main player for us since last season and under Ole as well, while Fred stance hasn't changed much at United since he joined, was a squad option when we had Herrera and he's still just a squad option now, and the only reason he played a lot in the first half and had time to gain some good form was because we had Pogba injured. Once we had all our main players back and signed Bruno, he returned to the bench. That's about it. The one who replaced Herrera in our main lineup was actually Bruno, as this is the only difference between the current midfield and the one during Ole's start as a United manager that landed him the job.
And as I said, we did a pretty good job building a great midfield 3 now, so it's not a problem that we let him go at the end. I have no problem with this. It's saying Fred is doing better than him or replaced him well is what boggled me that's all.