Clearly, it's a win-win; the recipient feels good that his or her success or gesture of kindness is acknowledged, and it distinguishes the writer for having executive manners. Best of all, it's fast, easy and sends a powerful message.
What super power do we need to thrive in this time of superlatives? You might guess extraordinary mental acuity or the ability to be in two places at the same time, but I think it's much simpler than this.
While the twenty-something-year-old in your family might be tempted to only communicate this way, your aunt, uncle, and grandparents certainly aren't. Set a good example and encourage "thank you" writing all around.