Exactly, it's because of those two CB's it's more important in central midfield to establish balance rather than all physicality or all creativity/mobility.
If you leave all the vertical play to your wide players as has been done in the past, it becomes to predictable. It was allowed to happen with the traditional 4-4-2's of old because there wasn't as much congestion in the middle third. But now that almost everyone plays with a three-man central midfield, there's less space to rely on wingers to move the ball vertically. With now having three attacking midfielders behind a striker, you cannot seclude them by having two less mobile "destroyers" centrally. There isn't enough exploitation of space because it's entirely too predictable, and therefore easy to shut down, passes to the attacking players. It's too layered, with not enough vertical interaction through the middle and therefore not enough balance.
Those old systems in which it was proven are not as relevant in the modern game. Look around the other successful clubs of Europe, and you'll note not one plays with two large destroyer type holding mids. The game has changed into a much swifter, more technical format. It is all the more relevant to us because of the stacked boxes we face at the narrow Lane because if you have such a midfield, you don't have a player to move the ball forward from deep and to get the ball to the forward players before the opposition can shut down the obvious supply routes. It's simple economics, as you have three sources of fast, forward, transitional passing rather than only two. Thus, your attacking mids have considerably more space to turn into and work with when the ball arrives at their feet. You're right that it's better to have two "holding mids" to allow the wide players more assurance in getting forward, but the key idea for balance is to have that more dynamic, forward passer also contribute in defense. Therefore there is no disadvantage, but yet more advantage.
