This is certainly not LUCK, but just one of several hidden and undisclosed ways Blizzard uses to keep customers coming back and still interested in the game.
Left for a long time? Blizzard make your card packs matter like you´ve just described.
Haven't completed new quests for weeks? Whenever I do that, I get the "Challenge a Friend" quest, no doubt the best quest in the game, reward-wise.
The current expansion is already getting old and interest is dwindling? You magically get better and better cards from packs, AS IF there was something like a "Magic FInd Property" (like MF in Diablo) to give you more LUCK.
The current expansion has just been released? That Magic Find will be at it's lowest and cards from packs will not yield that many Legendaries.
Can anyone PROVE I am wrong?
The question is rather: Can you prove you are right? A few postulates don't prove anything.