He left Bayern, because board didn't meet his demands. NT was his dream job as he was long time Löw assistant.
Pressing is the one of the main characteristics of modern football, while one of the most unsustainable either.
Pep uses it, and it was always ugly when it failed, Klopp usually had his teams taking season off from it, not just games.
One team that barely implemented it and succeeded is R.Madrid.
But they are historically the kings of pragmatic fundamental football.
Flick has massive flaw in being so dependent on pressing. This is who he is, and likely why no one (including himself) is expecting him to last more than 3 seasons here.
But while the discussion about sustainability is fair, the discussion about depth is equally important.
Flick is doing it with a team that lacks any depth, with many bench players who are trash, with many kids or older players, and he has fairly overachieved. And with a clown group of defenders in a club that ideologically doesn't care about signing defenders.
In that way, I would say "unhuman pressing" is his best bet, to cover for this shit defense. You defend by attackers and midfield for the most part.