I get that those stuck in the past think there was some special magic back then. There wasn't. Those weren't somehow bestowed with the designation of classic. Like everything else, it took time, which makes it no different than songs by today's artists.
When hearing Christmas music in general, (stores, radio, malls, restaurants...etc..) the classics (40's thru 80's) are mainly featured. Sure some newer acts are mixed in (like Beiber, Carey or Buble), but you mainly hear the classics...why?? Because nearly everyone is familiar when them, since they have been played for years. It has nothing to do with living in the past, it called music from the past, enduring the "test of time". 50 years from now, you'll still hear Bing's "White Christmas", a 110+ year old song by then.
If Beiber's "Mistletoe" is proven a classic, 20 years from now, then great...you'll hear that song and others along with the many pre-1980 classics then too. It's simply called "good music" and good music will last forever.