Huge part of that 3-1-6 was Eriksen's ability to play from the deep.. either to position himself as LCB pushing Shaw high up the pitch or to remain as a pivot when Casemiro dropped between two CB's. He did roam all over the pitch and was certainly the player with the most positional freedom, but a huge part of his offensive contribution was to help Casemiro and the team in build up phase.
So no, replacing Eriksen for Mount doesn't seem like a perfect solution even in that 3-1-6 in which we so often wanted to establish control and domination.
So, call me a sceptic if Casemiro - Bruno - Mount is our answer going forward.