An adult that still believes in Santa might not lead to anything bad, but it leads to them indoctrinating their children to also believe in Santa into adulthood,
And if some dude can live on the north pole and travel to every home on earth in one night, then other equally ludicrous ideas might not sound so far fetched
And before you know it you're wearing radioactive stickers to rebalance your chakras, sticking jade eggs up your ass to bring luck and you're blowing up a shopping mall because your imaginary friend hates gay people