Too many variables.
Martinez took over a well settled team.
Koeman took over a team that were a mess defensively but still had good players at a decent age.
Silva has taken over a squad stacked with bad players and old players that were once relied on (the latter being Baines, Jagielka, Coleman). As well as a group of players recovering from a season of Sam Allardyce.
I think it is media, experts and over-analysts which is making us leaning towards arguments like that. What squad did they inherit, Martinez was just lucky with the squad he had and destroyed it etc etc etc.
I for sure was one of the Silva-out shouters but probably just because my "dreaming" ambitions for Everton is bigger than the ambitions of the club itself. I do not believe that Marco Silva is the new deal, a new managerial star. I think he seems fairly ok. If we had appointed an extremely talented rising star of a manager you could just throw away all of the excuses above - as it actually just is excuses.
Look for instance at Pochettino. Number one - he totally transformed Southampton making them a top team before Tottenham went for him. So what kind of squad did Pochettino inherit?
Players like Lloris, Eriksen, Vertonghen, Rose and Kyle Walker was there but he also had to rely on Roberto Soldado, Adebayor, Capoue, Sigurdsson, Chadli, Sandro, Paulinho, Kaboul, Chiriiches etc etc.
This team was expected to win sometihng in the season before. In 2013 Tottenham splashed out almost 110 M pounds on players and recruiting Villas-Boas as manager. They ended up 6th - behind us. Next season they reached the League Cup final and ended 5th but most interesting Pochettino benched Soldado and other stars to make way for academyplayers such as Harry Kane. Lets hope Silva is of the same material and that Everton in two years time is on the right path with the right manager...I think both Silva AND Pochettino had pretty good framework for achieving success - as I said all of those excuses is just excuses, not real problems.