I think these thoughts are so important to consider, and perhaps help us wade past the shine of an enthusiastic offer from a program! I agree that the grad school application process is so difficult that it is hard to imagine that it is only the first of many challenges. I've spoken to too many current PhD students (from all kinds of institutions) who all entered graduate school thinking that, in this market, they would be the exception. It seems this belief in exceptionality is the rule among grad students.
All of the opinions gathered here (including my own) center in one way or another on the failed promise that PhD => TT job. Cobbling together advice that others have given me (and in particular the advice of one amazing professor), I wonder if the most productive way to make the decision for whether or not to pursue a PhD centers around two questions: (1) will I be losing something huge in the 5+ year process of pursuing a PhD (either mentally, emotionally, socially, opportunity cost in another career, or (!) financially); and (2) does the thought of pursuing a PhD sound sufficiently enjoyable to warrant pursuing it for its own sake. If you're going for that second question, perhaps add a third: (3) will I be able to drop out of my program if I find that it is not as enjoyable as I had anticipated.
I also think frequently of this New Yorker article by Joshua Rothman about the decision to pursue a PhD in the humanities in this job market. The link is here: https://www.newyorker.com/books/page-turner/the-impossible-decision. From this article (sorry suck at font formatting ? ).
What percentage of graduate students end up with tenure? (About one in four.) How much more unhappy are graduate students than other people? (About fifty-four per cent of graduate students report feeling so depressed they have “a hard time functioning,” as opposed to ten per cent of the general population.) To make a rational decision, [a friend] told me, you have to see the big picture, because your experience is likely to be typical, rather than exceptional. “If you take a broader view of the profession,” he told me, “it seems like a terrible idea to go to graduate school.”
And later in the article:
[...]there’s the fact that graduate school, no matter how bad an idea it might be in the long term, is almost always fulfilling and worthwhile in the short term. As our conversation continued, my friend was struck by this. “How many people get paid to read what they want to read,” he asked, “and study what they want to study?” He paused. ”If I got into a really good program, I would probably go.”