The idea that Mainoo is not a player capable of passing through the lines goes against reality but I'm seeing it in so many post I'm starting to wonder if people are talking about the same player.
When I watch Kobbie, I see a player who actually wants to pass through the lines more than anything. However delivering those passes constantly depends a lot more on your teammates' positioning that your ability. Him dribbling often tends to be a result of lacking passing options. We haven't seen him deliver long cross-field switches of play, but that is not harder to do than passing accurately with the right weight under pressure, things he has constantly shown.
In a set up with Bruno, we mainly need a person to cover the space in midfield. Getting another player in there more intersted in passing and creating will simply shift that physical responibility to Mainoo, and not only is he not that kind of player but that is likely to burn him out faster than anything else.