It's really disheartening that the culture has changed so much someone would think doing AoC puzzles just for the fun of it is an unpopular stance :(
Doing things for the fun of it, for curiosity's sake, for the thrill of solving a fun problem - that's very much alive, don't worry!
Eliminating the leaderboard might help. By measuring it as a race, it becomes a race, and now the goal is the metric.
Maybe just have a cool advent calendar thingy like a digital tree that gains an ornament for each day you complete. Each ornament can be themed for each puzzle.
Of course I hope it goes without saying that the creator(s) can do it however they want and we’re nothing but richer for it existing.