"how dare you fail to field a team that was as successful as last year after we sold your best player, a world-class striker!"
Yeah, this is not close to the whole story.
In terms of buying the groceries:
Koeman was vocal about wanting a striker and a left-back and the board did not get them. This is true and it's not on Koeman.
Koeman also pushed very hard for Sigurdsson, who will be the heart of the team for years to come, but the transfer saga dragged on for months and diverted the team from other targets. Most likely not attributable to Koeman.
I assume that Rooney was a consensus decision - and Rooney is all at the same time one of the few bright spots and one of the biggest problems.
Keane and Pickford were also consensus buys; at the time the fans thought that Pickford was an overpay but he's far and away their best acquisition and will be a bedrock for ten years. They might be below Palace without him (sorry jk).
Koeman scored a coup in picking Vlasic off of Hadjuk Split's roster after playing against him in the Europa tie. That is 100% to Koeman's credit.
Sandro and Klassen have been failures and those are entirely attributable to Koeman. The "Continental finesse player who can't handle the physicality of English football" is a tired and abused trope along the lines of "can't handle the pressure of playing in Boston," but in these two cases it's accurate. Klassen exacerbates the clog in midfield which is the biggest roster construction problem.
Koeman was a dick to Niasse, but Niasse isn't the binky that his proponents think he is, redemption story notwithstanding. Niasse is a poacher, but there's nothing to poach if your teammates can't get the ball into the box in the first place.
So on balance you'd have to blame the board more than Koeman, but Koeman hitting the lottery on Vlasic doesn't mitigate his overinvestment in Klassen and Sandro.
In terms of cooking the meal: Koeman has been a complete and utter disasater.
The lineup construction has been sheer guesswork, the formations and tactics shifting randomly from one game to the next or one half to the next. Understandable to a certain extent with so many new pieces, but the press and fans have been screaming for more width, more pace, fewer plodding No. 10's in midfield, and a stable defensive alignment. The shifts between back three and back four and quasi-back five (Koeman's favorite, with Baines and Coleman as wingbacks pressing forward around a conventional back three) have been without rhyme or reason. Granted injuries and the decline of Williams have left him little to work with (I'm not going to blame for the loan-out of Galloway, because he's been terrible for Sunderland). Rooney scores nice goals when the ball falls to him but that doesn't happen that much because he keeps tracking back deeper to get the ball. Last year Gueye was among the best DM in the PL, IMO, and he and Schneiderlin played well together with Schneiderlin as a holding-type DM in front of Gueye, but this season neither of them seems to understand what to do - Gueye seems to think that his job is to attack, which he sucks at, and Schneiderlin has been dreadful, continually being exploited on defense. It's been a fucking trainwreck.
As a leader of men, Koeman has also been awful.
He's vacillated between no-nonsense, take the players to task public persona, and making excuses for the tough schedule (either is understandable but pick one). No one seems to understand what they're doing out there, and the overall effect is listless, dull, tepid football. Everton's brand has always been that they would fight like hell and make the opposition work hard, especially at Goodison, even if they weren't very good. This year it's been... not that.
Roger Bennett posted this on the MIB Facebook page:
Watching Everton used to be a the joy of my week. A respite. An escape. An occasional wonder. Irrespective of the score, you were guaranteed 90 minutes of collective endeavor and tenacity, often in the face of stiff odds.
This season’s Everton are unrecognizable. The club have stumbled into a lethargic stupor of brain dead decision making in which the players toil with little evidence of tactical gameplan, cheer or care. To watch is to suffer grimly. A corrosive, toxic gameday experience without an upside. One that leaves any supporter, in a numb puddle of anger and despondency, and worst, short-tempered with all those around you whom you love. Manchester By The Sea, Johnny Cash’s “Hurt,” or Cormac McCarthy’s The Road lift the spirits more.
Yesterday was a busy family day and watching the Arsenal game was going to complicate arrangements. I told my wife before the game, "maybe I can spare the two hours; I can get the same net effect by smashing my ballsack with a hammer for thirty seconds." Oh how right I was.
There are enough good players that this team should not be in the drop zone in the PL and with one point halfway through their Europa group schedule. They have looked far, far worse than the sum of their parts, and even if it will take time to knit those parts together, and even if some of the new pieces and academy kids wash out, there is no excuse for the kind of rudderless, joyless, hopeless football that I've watched this season. The USMNT's showing against Trinidad is comparable in overall effect
but the USMNT's performance in Trinidad would be an aspirational level for Everton right now.
That is on Koeman.
So he had to go.