Here's my take: I consider a Christmas card a "token of love". A "gift", if you will: freely given without compulsion, with no strings attached. If I send someone a card at Christmas, on a birthday, to cheer them up, whatever, it is with absolutely no strings attached and under my own free will.
Any other condition, in my opinion, makes it become an "obligation", which, in my opinion, was never what "card giving" was intended to be.
The only time I ever gave out Christmas cards was when I was teaching high school, and I wrote out a card for each one of my students individually. Because I wanted to.
Do it because you WANT to. If that means only 5 people who are special to you will get a card this year, then so be it. If it means only 1 person gets a card, then so be it. If you don't WANT to do it, then it becomes an "obligation", which is not what it was intended to be in the first place.
That's just me. I've never felt bad about not sending out any cards. And if the people sending cards to me get offended because I never send one back, well, I question their motives for sending me one in the first place. And if I do send out a card or two, most people understand how special that is from me.
That's just me.