I suppose that the sentiment behind it is sweet, but it's so superficial.
How do letters or cards from perfect strangers compare to cards from people you know? Maybe it's just me, but what would make me sad about not getting cards is not so much the sheer number as the realization that I don't matter to people who matter to me, and strangers don't fall in that category.
Also, if this were to happen to me & I were flooded with wishes from strangers, not going to lie: I'd be freaking out that I need to reply to a massive number of people. And to tell them what? You need to add some coating to that "thank you", otherwise it's going to be so dry it'll sound forced (when in reality, it circles back to the previous point on strangers: what do you tell a total stranger? How do you build rapport?).
I also can't help but wonder how much of those initiatives are really about the recipient, and how much are about the initiator feeling good about themselves that they got their 15 minutes of fame. You know, until a dog who can yap backwards or a cat who walks on his hindlegs goes even more viral.