Oh, well white seniors, that makes sense. They’re the primary group watching Faux News and the war on Christmas is making them confused and therefore angry.
That is only true if one assumes that all involved are Christians or are expected to respect the Christian traditions more than any others. I don’t, and I’m not invalidating anything. My greeting is merely more inclusive.
ETA: Moreover, I don’t care how you say it. If you want to say, “Merry Christmas!” I’m fine with that. That does not impose on my any obligation to reply in kind.
Since I’m Asian, I’ve used “I’m Buddhist” a few times just to see their expression.
I did this once, but it only works before sundown on December 24:
“Christmas? Seriously?” (Checking imaginary watch) “But…but it’s still Advent. It’s not appropriate to wish someone a merry Christmas until the Christmas season has truly begun and Advent has come to a close.”
I’ll probably never do it again, and it WAS kind of jerkish. But it sure felt good.
When someone wishes me “merry christmas” or “happy holidays”’ I usually say “thank you” and walk away. They aren’t my holidays. I don’t celebrate any of them. But there is no point in lecturing a stranger about that, so I just walk away.
I do return “happy new year” with a “to you as well” or similar, tho.
Why jerkish? Anyone who believes that there ever was a “war on Christmas” is a moron, and deserves it.
I would go further. I would say, “Sorry, I’m a real Christian. We can’t say that yet–because Advent isn’t over. What’s your religion? Are you atheist? How nice! Tell me about that.”
“And does that mean you win?”
You could always go with Tolkien’s suggestion;
Well diffused by Bilbo of course.
Thumbs up, grin, no words.
Get indignant right back. “How dare you take the “holy” out of the holidays!”
It seems like this time of year has become so fraught with perceived offense and the desire to not offend that you can’t say anything without someone getting bent out of shape about it. Being rude, or snarky, or snippy back isn’t going to help anyone’s mood, and if the purpose of your “Happy Holidays” greeting is to be pleasant and friendly, then just wish them a cheery Merry Christmas back. Nothing takes the wind out of a rude person’s sails like being met with politeness. Or is there a personal reason you don’t say Merry Christmas? Then perhaps a better response would be “Festive Greetings to you, too”
Nuts to that: they aren’t my holidays.
Seriously, fight a little ignorance, take a moment to teach them that ‘holidays’ derives from the conflation of ‘Holy’ and ‘Days’. It’s meaning is actually, ‘Happy Holy Days’. And how can that be offence to a God fearing Christian? Then imply people offended by it are likely just a little uneducated or ignorant of its real meaning, and everyone else ought to take a moment to correct them, so they can stop embarrassing themselves like that, dontcha think?
But you aren’t encountering 1000s of people around this time of year, and presumably greeting them with something you hope is seasonally appropriate, as it seemed to me, the OP is.
Mazel tov.
The encountering 1000s of people, yes. The greeting them with something I hope is seasonally appropriate, no. 
If you don’t mind using a little Yiddish, the perfect response is “Merry Kratsmach” which translates to “Merry Scratch Me.”
Say feliz navidad. Then they can hate you for not talking American.
This is the winner. Smile and move on to the next person.