Her argument makes sense, although if Colbert the man had killed off the character, you can bet that he could have come up with a plausible (or at least funny) way of resurrecting the character if it were necessary.
As I mentioned, thousands of others have described the finale and/or their reaction to it and to the series as a whole. I'll confine myself to mentioning a few things that struck me only a day or two later.
Colbert wouldn't be human if he weren't a little sorry to say goodbye to the familiar surroundings of the past nine years, the place where he cemented his place in TV and pop culture history. He let that show for a few moments at the end of the episode as he stared into the camera. I've never seen him look so vulnerable or wistful.
After thanking "the Colbert Nation", he burst out with a disarming grin and a boyish, "That was fun!" It was obviously a completely sincere exclamation, and it gave just a taste of the warm and genuinely nice man who by all accounts lies beneath the Colbert character.
He continued, "Okay ... whew. Okay, that's the show. From eternity, I'm Stephen Colbert." Then he quietly put a bow on not just nine, but seventeen years of brilliance with one word. I won't spoil it; watch it for yourself. (Stay through the credits: not only is the theme music different (a Maureen Dowd piece might explain why), but there's one last recurring joke just before the very end.)
If you're cynical, you'll observe that Colbert got his cake and ate it too: he put his show to bed without having to kill off his character.
Maybe. But you know what?
The guy who pulled off one of the greatest long-running performances in TV history deserves it.