Most chefs are not cooking a roast dinner like they’re serving Henry VIII.
Neither am I. 
I've already said my Christmas dinner consists of the usual things that most people's consists of. Turkey, pigs in blankets, maybe some sausagement in the stuffing, maybe some bacon/pancetta in the sprouts. Meat juices used for the gravy. Meat fat used to baste the potatoes. All pretty standard stuff for a traditional Christmas roast. Flavouring things with the meat juices is not the same as eating excessive amounts of meat.
Most chefs are not sticking religiously to one flavour because of some weird tradition. I think you’ve misunderstood what it means to be a decent cook.*
Oh I think you'll find they are, actually. And it's not a 'weird' tradition. It's Christmas.
Besides, it's not about 'sticking religiously' to one flavour - wherever did I say that? It's about using the flavours known to complement those particular foods. I might have herbs and bit of lemon in the cavity of the turkey, celery and leek and carrots in the tray to add flavour to the stock which will later make the gravy,. I have cranberry sauce made with orange zest and maybe some Grand Marnier, I have sage and onions in the stuffing, chestnuts with the sprouts, honey on the parsmips, etc etc. Sometimes I'll do a spiced red cabbage with red wine, cloves, nutmeg, apple.
But the same way that I don't randomly add pineapple to my beef lasagne I don't add fresh chillies or parmesan cheese or anything jarring or unnecessary or unauthentic to my Christmas roast, just for the sake of being different. I don't want 'different' on Christmas Day. I want familiar. I want nostalgia and tradition thanks very much.
I eat 'differently'' enough all of the rest of the year.