Not necessarily translatable in this instance. Since the game is owned by Inno, they have full control of the content. In addition, the GTC says that they are even allowed to completely end the games under their license(s) at any time at their discretion. So you could theoretically buy 10,000,000 diamonds worth of stuff, then the next day Inno ends FoE, and you wouldn't have a legal leg to stand on since there is no breach of contract. The question in a lawsuit if you were to bring one here would be whether or not a reduction in attack bonus from 5% to 3% is a cognizable tort. In all likelihood, the judge would rule that the in-game materials have no quantifiable value, therefore you have no case. The problem with your "kilo/half-kilo" analogy is that you're not buying a material good here; you're buying a service-specifically, entertainment. A better analogy would be if you bought tickets to see a baseball game, you take the whole family, then when you get there the star outfielder has been benched for the day so your kids don't get to see him play, and then the team loses on top of it. Disappointing, sure, but you can't sue to get the cost of your tickets back. Or another example, let's say you take your kids to see their favorite band, and it turns out the band sucks live. You can't sue on the basis of "this isn't what they sound like on the album".
I agree completely. I'm not saying the scenario I listed above is ethical. I'm just giving my best guess as to how a legal case would actually play out. You can't sue for a reduction in quality of a good that doesn't exist, especially when that "good" is completely at the discretion of the company per the GTC, which everyone playing the game must agree to in order to play it. It's a cruddy deal for the players but you're not going to get anywhere in a courtroom with it. The Oracle of Delphi fiasco was an excellent example of this. The owner of an intellectual property like a game can retroactively change it at their discretion, provided the contract between the entertainment provider and the customer doesn't prohibit it (which in this case it doesn't).