It was over 10 years ago that I contacted Glenn Morton. Aside from the question of the age of the earth I wanted to know why he had caved to evolution. How did it follow that an old planet elicits that kind of capitulation of first principles regarding faith and the clear written text of Scripture? Sadly, nothing in his reply back to me answered my question. He, as most do who turn away from YEC to evolution, avoided a real answer and tried to bring the discussion back around to an old planet.
That is why @thoughtful is probably very near correct here. Somehow when YECs begin to express doubts about the age of the earth, their faith falters and …strangely (at least to me)…then feel somehow compelled to go ‘all in’ and embrace an evolutionary paradigm.
I am still perplexed and completely baffled at this kind of reasoning. Evolution simply does not follow from belief in an old planet. Glenn Morton thought it did. I can only guess that he must have reasoned that he had no choice in the matter.