There's no doubt everything leads back to the board. Everything. Their recruitment has been disastrous - including, in general, their recruitment of managers.
That said, there is a minimum standard any Everton manager must meet at any given time. In the days of Silva, given how he was supported financially (albeit by the disastrously ineffective Brands - see? Recruitment by the board was disastrous at all levels), he really could not complain if he lost his job losing to the likes of Millwall and getting absolutely hammered at home by Spurs and away by Liverpool - among other atrocities over a 12-month period (such as home defeats to Sheffield United and Norwich). Anybody can lose a cup game to a lower league club - but you can't do that and see your side obliterated by the clubs you are apparently chasing having spent a fortune while being unreliable against clubs with a fraction of your resources.
Looked at from today's predicament, people might say he was harshly treated. But standards were higher then and he didn't meet them. We can't lament the constant decline in standards and say we should have given Marco more time. By the end, he looked like a tramp who'd spent a wet night under a bridge.