I've never seen anyone get offended by "merry christmas"
And this is why it's ridiculous that "happy holidays" ever became a thing in the first place.
It probably has more to do with Christmas not being the only holiday celebrated at the time and advertisers trying to be more inclusive and not alienate a potential market segment. Seeing as how Christmas is way more commercial than it is religious, the opinions of marketing execs carries more weight than anyone else.
How big is the market for Hanukkah and Kwanzaa anyway? You're not making a very compelling argument.
Don't know, but it's more convincing than people clutching their pearls because they heard merry Christmas. ---