If Pogba - Pirlo - Vidal worked I don’t see why Pogba - Arthur - McKennie wouldn’t work, and keep in mind that we still have Rabiot and Bentancur.
I think the difference is that Pirlo and Vidal were world class. Vidal with us was like 2 players on his own, he could do it all. Pirlo was one of the best players in his generation, he could pick passes nobody could and score goals.
I see what you are saying that Arthur could be the regista and McKennie the energy but they are both significant downgrades on Pirlo and Vidal and not as rounded.
The other midfielders next to Pogba (Marchisio and Vidal) could do multiple roles. Unfortunately all of our midfielders are less rounded and seem to specialise in 1 or 2 areas but weak in others (Arthur = good ball retention but can't create, defend or score goals, McKennie = good movement and energy but bad passing/positioning, Rabiot = Good carrying but can't create or score, Bentancur = probably the most rounded but needs to add more in an attacking sense).
I'm not against Pogba at the right price but he is far riskier than a lot seem to think and would take some reorganising to fit him in. At the moment you'd need to play him where McKennie is but then we lose his press. You could move McKennie to one of the pivots but pressing like he does from there is crazy (eg Roma game) and I'm not sure he has the positional sense to play there.
Like you say a midfield 3 suits him best but you'd need to find balance somewhere to protect the defence.