There's one fatal flaw in this author's logic.
He uses self-identification as the arbiter of what is considered liberal and conservative. Yet those numbers are largely static. If the country as a whole moves left, the perceived "center" moves with it. So, consider 20 years ago someone who embraces the entire alphabet soup lobby agenda would be considered a hardcore leftist back then; now, that's considered normal and even personally objecting to taking part in it is considered "discrimination."
There are some other issues, too, but that's the one that most easily sticks out.