This. I’ve always found Happy Holidays to be a very appropriate thing to say. I’ve Jewish and Islamic friends who don’t celebrate Christmas, so saying Merry Christmas in a context where I’m wishing both my Christians, Secular, Jewish, Islamic, friends and acquaintances a good holiday season would be wholly inappropriate. Nevermind those I don’t know who may or may not be Christian.
Now in a context where I know everyone I’m speaking to is Christian (or secularly celebrating the modern secular Christmas), sure I’ll generally say Merry or Happy Christmas. But in most contexts I live in a diverse place and I’ve always felt it’s more appropriate to cover my bases, and Happy Holidays does that.
By the same token however if someone were to say to me Happy Hanukkah or Happy Kwanza or something along those lines I wouldn’t take offense. I’d be glad someone was sharing a holiday sentiment with me and leave it at that (as I hope they’d do if I wished them a Merry Christmas and they didn’t necessarily celebrate the holiday).