The discussion is why can some players who are a 10 or an inside forward able to play as 8’s (Bernardo Sliva, Havertz, McAllister, De Bruyne, Foden, Bellingham) examples off the top of my head (I know McAllister is playing 6 atm) and others can’t? How much of it is tactically or position inflexibility or the manager not setting up the team properly.
And yes I am wandering my Mount or Bruno can’t play as double 8’s like the rest mentioned can.
Bruno, and to a lesser extent, Mount, are not positionally disciplined enough to play the 8 role.
There’s a huge difference in playing the 10 versus playing the 8, which is usually the 10 are given a free role to roam in many teams (offensively). This suits players like Bruno who is excellent in the free role but is unreliable in the modern positional game that require many 8 to occupy exact spaces in out of position phase.
All the players you list who can play 8 is positionally and defensively solid. With sufficient coaching, Bruno and Mount can succeed in the 8 role but they are not quite there yet.