People are obsessed with 4231 vs 433 as if playing a 433 means you can justify having a completely imbalanced midfield.
It's possible to play an 8 that has qualities that complement our other midfielders, and a lot of fans are just not convinced Mount is that, and they said as much weeks before we even signed him.
We ideally wanted a player who makes a high volume of passes while progressing play with very good ball retention. One of Mount's problems has always been he's a periphery player, he isn't that involved in the game and doesn't make many passes compared to other players in his position.
Neither Bruno or Casemiro are strong ball carriers, so it would be useful to pair them with one. Mount isn't.
Bruno lacks physicality, and so we pair him with Mount who has the same issue. Casemiro has lack of recovery pace and aging legs, and we pair him with a player who's spent his whole career as an attacking player with little evidence he'd be effective at helping Casemiro with the defensive work to the level we need.
Playing two 8s isn't a function of the player themselves, rather the instructions the players are given and the positions they take up on the field under the guide of the manager. Fred, Eriksen and Mctominay can all be our second 8 alongside Bruno if that's how we want to set up, but clearly they all have different strengths and weaknesses. Some are going to give you more physically, some are giving more technically, some are giving more defensively, some are giving more creatively. It's about finding the right mix of qualities in your midfield 3 and I suspect we don't have it.