For the most part, it was a glorious exhibition of what could be. Everyone bought into it: fans and players and media. But it couldn't last. It was almost too good to be true playing like that and winning 21 out of 38 games on the relative resources we had in comparison with the financial elite (we did it with a lot of loan players, for example, which pissed the top club loving media off no end).
If I was to put forward a reason for why it didn't last much longer after that, I wouldn't spare the rod with Roberto. He gave the impression (and I said it at the time) of a man who thought this business was a piece of piss. It isn't. You have to get better each season and build more and more strength rather than rest on your laurels...which is what he did by his World Cup 2014 escapade and subsequent no show pre-season. That was compounded by going all out in the EL competition early doors rather than treat it like a nuisance until the KO stages (RM no doubt wanting the rest of Europe to see how much he'd developed himself and us).
All told though it goes back to something I feared right at the beginning when he arrived here: the task he had was monumental in that he was trying to shift a culture that'd grown up even before Moyes arrived where fans and players were edgy and comfortable only with a direct game. That proved insurmountable when we hit hard times and he wasn't a man capable of going to an alternative plan to get through those hard times. I get that: he was/is an idealist. It didn't come off for him though.
His legacy is that we could never have attracted Koeman with a squad pf players who just had a long ball game. Koeman might now state how he's managed to solidify and add stamina to the team as a defensive unit, but he knows that this team can lay when they get on the ball, and its easier to come to a club and sort out the former than the latter.