I do agree that Blizz needs to abandon their approach of only making advanced announcements for things that deviate from what is presumed to be their standard operating procedure. It is silly that they will announce that Residium will not be before that was really ever a concern, but they do kind of force you to assume XYZ. Only because, on occasion, people have been burned by assuming blizzard would not allow certain things, based on previous efforts on blizz's behalf, but ended up allowing them, or vice versa.
However, I do think it only really impacts an incredibly tiny amount of the population. Also, to be technical, those that did do this haven't actually lost much of anything. The only way they have truly lost anything is if they by setting themselves up like this, that the actually wasted Rep gains by idling at 19,999, and therefore progress being lost to a 4th potential cache. Blizzard removing the AP from the cache itself after 8.2 doesn't negate the work they did for the cache, since it will still exist and still have almost all the possible rewards in it. What does alter, is the opportunity cost of them doing so, since they planned their current course of action with their assumption that would get the AP from the cache's they've turned in, whereas they could have just grinded out 19,999. Ultimately, I believe this is what players are likely most upset about, whether they realize it or not. The time spent on the those 6 cache's could have been spent maybe grinding out island maps until you can say, have 2 missions pending completion that reward large chunks of AP, or whatnot.
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You know the intent isn't to hoard them because A.) you can only actually have one box at a time, and B.) you have a strict limitation on how many pending cache's you can have. I Blizz intended for you to be able to actually hoard boxes, not to just have one sitting in your bags you choose not open, but to stockpile them, they would have no such restrictions. Also, because they have done this exact thing in the past, and expressed their feelings about this exact kind of thing in the past.
A paycheck is also a terrible comparison, but if we want to go with real world stuff, compare it to sick/personal days. Many places will allow you to rollover left over days year to year, and most companies have limitations in place for how many days (or hours for some places) you can rollover. Someone might decide they are going to save up as many days as they can, and towards the end of the year, try to say, take a month and a half off. Most companies will not allow you to do that, and will instead recommend you use it in smaller blocks. Just because a system theoretically could allow for something, doesn't mean it can or will, especially when patterns indicate it will not.