Well I wouldn't say the 2 8s aren't also an issue because, the two we've seen the most, Bruno and McTominay, have their own downsides in terms of limits in their skillset. But overall, you are completely right, it isn't a new thing at all and it isn't the system or formation we are using per se, it is our execution of it - a) the actual understanding of the system, who is doing what when, b) synchronisation, if player a does X, I have to do Y asap and c) overall subpar workrate. The system ETH has chosen this is ambitious, knowing our squad and from where they come from but it isn't wrong per se.
I think, people are way too hungup on the single-DM thing. I mean, the observation is correct, we aren't doing a good job of being solid in this area but it isn't like the system is just created like that. Just look at City, just in the last like 3 years they came up with multiple ideas to add personal to the middle in certain situations during a match. Be it a fullback who tucks in like Walker or Cancelo or be it a CB pushing up like Stones more recently. Pep and many other coaches try to create situational formations, against the ball it looks different than when on the ball. But this requires players to have a good understanding of their roles, a good general intelligence, round skillsets in a lot of positions and good workrate. For all the talk about the pros and cons, all our midfielders of the last years weren't really rounded in their skillsets. Yet you have seen posters claiming that Bruno and Eriksen should just play the 8 position. Now we finally have a few midfielders who can be trusted in midfield, Casemiro, Amrabat and Mainoo can contribute on and off the ball. Casemiro isn't one for the future, Amrabats overall level is probably a bit on the low side and Mainoo is young. But we are getting there. From what I've seen, even Mount is a bit more rounded than Bruno or McTominay.
And this time it isn't me saying we need to get rid of Bruno for it work, we just have to make more of an effort to balance it out. Casemiro, Mainoo and Bruno sounds pretty good on paper to me. Then either Martinez to push up or Dalot to invert (theoretically Shaw should be suited to do that as well) and a player with Antonys work ethic but Garnachos output on the right plus Hojlund with more service and more class in front of goal and we are set as a functional team. But it will take some for players to bed in. Injuries haven't really helped as they prevented synergies to emerge. Somebody said, the level of fluidity is a bit too high, I am inclined to agree. It seems to be a little to much to handle for some of our players.