It is fantasy and is not about players covering for one another or fluidity.
No team will set up with a sole DM that has a box to box role and freedom with it.
All you are doing is restricting FDJ and taking away freedom to play in way that suits him best.
That would be the Bilbao game where FDJ/Pedri helped dominate and you want that midfield dynamic to change to put FDJ back to DM. Aye same game.
Let me explain.
The DM will have moments where he can push up. For example, when not pressed and presented with some space, he will not hold a position like a robot. If he has the chance to gain space easily and draw markers (thus unlocking a teammate above him), he will have to advance with the ball. In those moments, one or more of the players near the DM will have to make some adjustments as not to leave the space the DM vacates open and vulnerable to counters when opponents appear in that space.
Like here for example. How do you explain that the DM finds himself in a more advanced position than both Messi and Iniesta, the "10" and the "8". Both of which are far more attacking players. The answer is "because he can". He is in the position to use space, and advance the play, and he does. The position you play in doesn't stop you from doing things when opportunities present themselves.
Your comments don't account for the vast amount of inter-positional movements that happen in football. When you play DM you can still move up, to generate dynamism and superiority, exploit empty spaces, and all that stuff. You just do it less than in more attacking positions, because you have other, more important roles, and not least because the opponent doesn't allow you to.
