I'm not even sure what this nebulous "window is shut" is referring to. What window? And how exactly is it shut? Window for....having a decent team? Having a playoff team? Getting to the SB? That's all based on pretty subjective criteria. Having everything in place guarantees nothing.
It's a parity driven league and it all matters, talent, injuries, luck, coaching and on and on. The margins are thin and one key injury can derail a season for most teams. KC is one of the few truly dominant teams out there. Coaching has something to do with it, but their talent level is also undeniable.