Yep, you are right - the phrase, 'ruin the league,' is a bit of an overstatement, but you get the point. The last 4 Finals' have had the same two teams. It's not simply that an all-star chose a new FA destination; it's that due to an unprecedented rise in the cap two years ago, the best team was able to get one of the top 2-3 players in the NBA. It's especially frustrating to a team like the Cs who, due to DA's incredible cap management, would have been able to add Horford (or Durant) regardless of the spike.
Also, take into account that GSW was able to sign all of their star FAs to pre-spike long-term max contracts and it created the perfect storm of sorts. I understand it from GSW's perspective (obviously, they weren't going to say no), but they were already awesome, they didn't need - or, really, deserve - Kevin Durant.