The last time I checked most people in this country celebrate Christmas
How do they celebrate Christmas? Is it by going to church/mass? Or is it by covering their house in twinkling lights, decorating a tree, baking cookies with their families and telling their children that a big fat man in a red suit snuck down their chimney in the middle of the night and left them presents? That latter version of Christmas is hugely secularized already, to the point that the entire religious meaning of Christmas has been replaced with an inundation of "Only __ shopping days left until Christmas!" before you've had a chance to digest your Thanksgiving feast. You want to talk about the war on Christmas? It isn't the people saying "Happy Holidays" who have changed the meaning of the holiday, it's the marketing executives who have done a bang-up job replacing Jesus with Santa over the past 60 years. Christmas, the celebration of the birth of the Christ, has turned into the biggest commercialized shopping event of the year, something which Jesus probably would have abhorred. The "War on Christmas" isn't someone saying "Happy Holidays," it's the replacement of your messiah with products, and it's a war that was lost a long time ago.
But, yes, clearly it's the "Happy Holidays" that really changed the meaning of Christmas.