Why ? Maddison is a different kind of player - and is probably the player in the P.L that is most similar to Bruno. What I dont like about Maddison is that he isn't a really quick player - and we have too many of those in our side (Casemiro, Eriksen, Bruno) - Mount is rapid and really good at driving the ball forward with pace. That is the kind of players we need, because we haven't got it.
If you remove the stats from last season - his numbers are 105 matches - 24 goals and 20 assists. And then we are (as you say) at 15 goals and assists pr season in his first 3 seasons at Chelsea. And he does that without set-pieces
Bruno Fernandes has 44 goals and 33 assists in 124 matches - and I haven't bothered to look it up but I assume close to half of those goals have been penalties. So maybe Bruno has something like 25 goals and 33 assists in 124 matches not counting penalties - so he really isn't that far behind Bruno. And Bruno is by far our most efficient player behind Rashford.
Marginally upgrade from Fred ? Marginally ? Fred has 15 goals and assists in his entire United-career. As for Eriksen, he was a point-player in his early Spurs-days - but in his last 3 seasons in the P.L (last season with Spurs, Brentford at United) - he has scored 5 goals in 70 matches. These days Eriksen is a playmaker - he will not score or assist a lot apart from the odd set-piece.