I tend to agree with you. I always use Shanahan as the example since he is seen as one of the best coaches in the game right now.
49ers record with Purdy or Garoppolo starting: 55-21 (.724)
49ers record with any other starting QB: 9-30 (.231)
Change one player, and it's the equivalent of a 12-5 team compared to a 4-13 team. Coaches of 4-13 teams get fired. Of course, other players on the team are affecting the win totals, but Garoppolo was winning with the same teams that Mullens, Hoyer, Beathard, etc. couldn't win with.
Garoppolo record when he started less than half the games in the season: 9-5 (.643)
Other QB record in those same seasons: 7-27 (.206)
He had more wins with Garoppolo starting in 14 games than he could pull off with the QB carousel in 34 games.
It is fair to say that good coaches and good players make each other better, but coaching will not get it done alone. You may be able to string together some tough victories, but being a legitimate contender year after year can't be done without talent.