Gibson isn't as good as McCarthy at keeping fit, for starters, or at being younger, or at being mobile.
Maybe not, but Barkley is. So's De Guzman, Diame, Fer and a whole host of other options that WON'T cost the £10m+ price tag attached to McCarthy. He's young and English and therefore there is a premium attached to his sale price.
Put another way, Honda is likely to go for less than McCarthy: that's insane.
As for the mobility knock against Gibbo, have another look at the last couple games this season, particularly West Ham at Goodison, where Felli was played in CM instead of 10. Gibbo had much more freedom to roam with Felli close by and took the opportunity. He can't do it with Osman alongside him because Osman has no defensive presence. Stick Fer or Diame in CM and you'd see a mobile Gibbo.