As for that second bolded part, the difference in how our players have done under capable management or without is massive.
Under Kovac, Müller looked like he was finished and was on his way out of the club. A textbook professional and one of the club's greatest players in history, it got to the point where his wife was complaining about him being subbed on Instagram, something like that never happened before and never happened since, he was booted out of the NT, he was poor in form. Look how Müller has been again since the second Kovac was out and Flick took over, he's been a top 5 attacker in the world, integrative leader on the pitch and in the dressing room. Kovac frustrated the shit out of players.
In the 00s we were a patchy, disjointed, in parts poor squad. The impact of Van Gaal who brought a coherent playing style was massive (even if he was too flawed himself to be really successful, as you guys are aware, and even if the squad building was a long and winding road; van Gaal for example was against buying Neuer, the absolute buffoon). But we didn't row backwards after him to hire Mourinho, we hired Heynckes who explicitly said at the time that he doesn't have to change the fundamentals of van Gaal's imprint, just the balance and add pressing (and not being a narcissist stubborn twat like Van Gaal helped, too). Contrary to Mourinho claiming he needs to coach the Van Gaal stuff out of the squad. Our club doubled down on a possession based style and Sammer as DoF in line with the board worked to implement it all across the club (hiring Ten Hag for 2nd team, for example). When Kovac was critized by the board for his work, publicly, it was not due to results but due to abandoning the principle of dominant, attacking football that led to poor results.
Your managerial appointments were both underwhelming and poorly conceived. You also had bad luck of timing with available candidates. This can all change with Ten Hag.