There can be no argument that the Texans have improved every year under Kubiak's leadership. Since they dumped HWWNBN and traded for Matt Schaub in 2007, the Texans have finished finished .500 or better all three seasons.
Here's the list of teams that have done that, with wins in reverse chronological order:
Arizona 10, 9, 8
Dallas 11, 9, 13
Indy 14, 12, 13
Minn 12, 10, 8
NE 10, 11, 16
NYG 8, 12, 10
Phil 11, 9, 8
Pitt 9, 12, 10
SD 13, 8, 11
Tenn 8, 13, 10
Hou 9, 8, 8
Of those teams, only Jim Caldwell, Mike Tomlin, and Ken Whisenhunt have less NFL head coaching experience than Gary Kubiak. Yes, Gary Kubiak is the only coach on that list that hasn't made the playoffs, but I don't think it's completely fair to judge his performance as a head coach based on that fact alone. He has done a good job overall or the Texans wouldn't be included here. On the other hand, he's failed because the Texans haven't cashed in on their potential yet.
I don't think Kubiak's coaching or leadership skills are eclipsed by Ken Whisenhunt, he's simply a victim of circumstance in the form of the AFC South. Put the Texans in the NFC West and they win twelve games a year. The AFC South is the best division in football. It frustrates me that this team can't win those important divisional games, but I'm not convinced that a head coaching change would rectify this. Players gotta lead on the field and learn how to close out games.