It is my patients, mostly, who say “Merry Christmas,” instead of “Have a nice holiday.” It is the staff who says the latter. I don’t think psychiatric patients are more likely observant Christians than average (though just about any group would be more Christian than the staff at a psychiatric hospital); I think it is because they are in less contact with the political correctness of the surrounding society. Theirs is the older response, the one they grew up with – they haven’t changed.
Shopkeepers are quite careful to stick with some “holidays” formulation. One exception is the place where we cut down our tree each year. Without having to ask any specifically religious questions, it does seem a safe bet that a person buying a Christmas tree would be a little petty to object to “Merry Christmas,” even if their tree marks only a secular version of the holiday. People who claim to be buying “a tree for Solstice” or a “Hanukkah Bush” will have to just endure it. They are rationalizing anyway, buying a tree that fits the celebration of the surrounding culture, based on its overwhelming Christmas orientation, and just renaming it. If you’re that opposed, don’t buy a tree.
Who does actually say “Merry Christmas?” People who are aware I am a Christian, but often only if they are Christian themselves. Only rarely do I detect an aggressive note to “Have a Happy Holiday” from a person determined to remind me not to dare impose my “Merry Christmas” on them. Mostly, people just install an automatic saying at the beginning of December and just go with that throughout. Most people would likely most want to say what you’d most like to hear, but that takes a lot of energy to size up fifty greetings a day.