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.
This can all be summed up nicely in one word: jealousy.
Durant did nothing wrong, nothing dishonorable, and in fact should be lauded for his willingness to sacrifice money to chase after his goals. Instead, a league of jealous fans condemn him when had it been their team in the same position would be signing a totally different tune.
Yep, totally jealous. I love the position DA and the Cs have put themselves into, but I would gladly trade positions with GSW (including their 3 recent championships). I would love to have a 20-16 lead over the Lakers right now.
But I disagree that it's all about jealousy. A top player joining up with an all-time great team after your team was knocked out by that team is a little 'dishonorable.' And, even though Durant has proven that he is indeed weak with his [somewhat] recent Twitter activity, I am sure he knows that history will be on his side in a few years when he has 5-6 championships next to his name and people are calling him a top 10 all-time player because of that hardware.
But GSW wasn't an all-time great team. The year they set the record for most single season wins they didn't even win the championship and were beat by a better team. People can make up whatever excuse they want for why GSW didn't win that year, but the undeniable fact is they didn't win. In the two seasons prior to Durant joining GSW they only were able to beat the Cavs when Kyrie and Love were injured.
Heck, even last season when GSW did have Durant, Houston had a better record and was one Chris Paul injury away from beating GSW in the WCF. The bottom line is GSW has absolutely needed Durant to win the last two titles, he was the piece that put them where they are, and if Durant wasn't there it's very possible GSW still only has the one title from the year they beat an injured Cavs team.