Obviously the manager/coach matters, but modern football is overly obsessed with the idea that these people come with a 'philosophy' that they 'implement'. As if they are a chemical that you apply to achieve a set outcome.
A manager is one part of the complex, multifaceted and variable equation that is a football team at any given point in time. They matter, but so does how the club is run, by whom, to what purpose, the mindset and quality of the players, the chemistry that either does it doesn't become generated somehow, the specific expectations from inside and outside the club, luck, etc and so on.
A coach who produced a successful team at one club one year might, despite adopting the same approach at a different club with better players, not succeed. We've seen it time and again. Yet there is so much money riding on football that the people who run clubs want desperately to believe there is such a thing as a reliable method. That they just do their homework correctly they can be assured of the outcome in advance. Nonsense. The proof is found in the pudding (or more often not found) again and again. It is impossible to identify a 'right' manager for any club in advance, only afterwards.
The media have gone from barely understanding football tactics and data thirty years ago to analysing it to death, to the point you'd think it was an equation, or a chess match between two managers rather than a fiercely contested sport of fine margins. That just places further undue emphasis on the manager and his tactics. Unless he's a hopeless clown the fate of the team doesn't all turn on him, only part of it does.
We don't know what would have happened if we'd kept Glasner, or got Iraola, but neither does anybody else because these things just aren't that predictable. I don't think we can ever say we dodged a bullet or missed out. We don't know how things would have gone.