Disagree about that, he's one of the very few players I could imagine at DM for us. He has the same imposing presence Busquets has/had (for a lack of a better description)... unlike Frenkie, who has no real presence in front of the defence. If we had two physical/fast CB's I wouldn't rule out Frenkie in a classic Barca DM (falling between the CB's)/libero role though - he was fantastic there vs PSG, probably the best he has played for us. Problem is the 2nd CM then.
About CB's at DM... it can work with mobile guys like Mascherano/Marquinhos ... but Garcia is quite stiff.
Tchoumeni doesn't connect defense to midfield which is huge requirement for a CDM in a single pivot.
He has defensive abilities, although still not perfect, but on the ball he won't do something that is essential for single pivot system. It shows in both eye test and states btw
What makes things tougher is lack of CM that can do that part, FDJ isn't the guy, and Pedri can be but he is more advanced player, so it will be tough for a DM of that category to help midfield. When we had Xavi, Iniesta & Deco, a Marquez or Edmilson are fine, when you have a Modric and Kroos, a Casamero is fine.
He will be upgrade over ancient Busquets, just like Frenkie is an upgrade over old Rakitic, but it will be same story.
For the price and hype,he will always leaves us wanting more, and not feel he is long term answer.
Edit: as for Eric, I am not sure he is more stiff than Edmilson, it os something that is worth a shot on training but but going by Xavi shutting it down, doesn't seem it was success lol