People are too obsessed with 433 vs 4231.
Whatever the set up, you need a mix of qualities in your 3 midfield players - a player with very high ball retention and a high volume of passes, a player who can run with the ball, a player with recovery pace defensively, a player with physicality, a player with creativity, a player with goals, a player who can tackle, a player who can cover a lot of ground.
We have some of the qualities, but we're severely lacking in others. Bringing Mount in to partner Casemiro and Bruno will likely leave us lacking physicality, defensive stability, recovery pace, and none of them are really a player who can set the tempo by making a lot of passes and reliably keep possession whilst progressing the play up the field.
It also doesn't help that our ball retention is horrendous all over the pitch so we are gifting the opposition a chance to hit us in transition far too often. If you don't give the ball away then you can get away with not being as defensively solid in midfield, but when you're constantly losing the ball needlessly the opposition will have a field day.