You just completely countered your own argument, right there though. 21 games as a LM scoring 4 goals, assisting 4, improving this teams possession with the hightest (tied with Melo) for passing completion he did nothing but improve the overall teams performance BECAUSE he did the simple things right. Unlike, Sissoko for example, whom had to always win the ball back after losing it himself and not making the simple decisions and actually completing them.
People like to overlook the hard work Marchisio puts in, tracking back, marking, cutting off channels, goals, assists, his technical ability to pretty much be competent across the entire midfield. Selling Melo and bringing Vidal is a step up because we are getting a player that apparently has it all. Melo is solely a destroyer and aren't you opposed to that type of play anyways because these types of players don't really do anythng for teams possession? Sure, if we sold Marchisio for a better player then I'd be for it too. He is no WC CM, obviously so if it time came to improve his position then by all means do it. This is what we are doing with Melo, so how can anyone have any beef with it? Just because he was our "best" player last year? An upgrade is an upgrade, doesn't matter who it is.