I think they should be seasonal, so over the same period that reindeer provide their boosts. They are much more common then as people use up the items, in particular the ring which once you die with it can’t be replaced. They are much rarer the rest of the year so won’t be missed. And anyway it’s pretty obvious when someone is using the set as trees appear every few seconds.
They could add a switch, but if they did it would make sense for it to work for other sets, such as ST tier item sets, the Valentine archer, etc. . And i don’t think that fits into the design of the game – the whole point of their 16 × 16 skins is a reward/recognition that you’ve completed and are using the full set.