The idea that MGW comes in and instantly improves us because he offers legs is wrong. Even highly intelligent players like Rodri and Bernardo needed a season to get Pep's positional football. You may be the best athlete in the world but if you don't understand how the team moves in attack and defence, when to pass the ball sideways and when to take risks, your athleticism is pointless.
Football is a simple and great game to watch, but creating a top, top team is utterly difficult. If we fans think of midfield play in 4 or 5 parameters, Pep thinks in 14-15, realising that there are hard to calculate trade-offs regarding technique, intelligence, power, stamina, speed, defensive ability, being dangerous in attack, etc. There are so many moving parts. There's no simple algorithm for success.
In short, those thinking MGW would instantly improve us if played over Bernardo are a bit clueless. Now that Bernardo may start once in a week, his performance levels are likely to improve. He's, what, 3 years younger than Salah.