This is a great point, you can certainly be a phenomenal coach and never make it out of D3 because you lacked arena management, training plans, roster building, market pricing/timing, and a whole load of other things.
I think the only real counterargument I'd have here would be that for a team playing at the top leagues, tactics mean a lot more than they do in D2 and below. You can steamroll through D2 off almost team strength alone if you have the right team. Using amazing financial management and training to overcome your inability to coach is a possibility in those divisions.
Then you get to a point where you have to be good at everything once you get to the very top. So in terms of proven coaching ability in my eyes that's why I take league success somewhat into account.
2x NBBA Champion: S55, S56. 3x USA Cup Champion: S54, S55, S56. WR for longest home streak ever at 11 in Season 47.