Question: The issue has crept up on me slowly over the last several years. I am increasingly aware of the massive debt that many of my students are taking on, debt which is far beyond the sort of debt that I incurred as an undergraduate in the 1980s. Because of this, in recent semesters I have found it somewhat difficult to fail students. Instead of simply asking myself "does this student deserve to fail this class?", I find myself asking "does this student deserve to have their life ruined?" In many cases (e.g. students who are already on academic probation) this is not much of an exaggeration. It is a very bad situation to find yourself in your early 20s with no college degree but $30,000 in debt. In some cases, I am aware that a decision of mine might be a contributing cause of a student ending up in just such a situation. I can no longer regard a failing grade as a relatively minor matter (like a speeding ticket).

How do professors reconcile their de jure role as guardians of academic integrity with their de facto role of being (at least in part) responsible for their students' economic future?

Answer: 
Take the example of a medical student. Do you want to pass someone who does not have the necessary knowledge to treat patients correctly? It is your duty to make sure that only the ones who know what they are doing will pass. This may be less strict in other subjects but the principle is the same.

--- EDIT ---
Another example where this becomes clear would be an airplane engineer or pilot that does not have the necessary knowledge (thanks to Mike's comment below!).
Here's an added perspective: If you start passing students for non-academic reasons (or really even just start spending mental energy considering that), there is *no lower bound* to that concern. Wherever you decide to set your threshold, there will always be students further down academically. There will always be some students to be "ruined", and the more your institution lowers standards, the further-down your population will drift over time, in like response to the lowered expectations. 

This is notably coming from my position as a faculty member at a (large, urban, northeast US) community college. Nationally, [community colleges only have a 22% graduation rate after 3 years](https://www.communitycollegereview.com/blog/the-catch-22-of-community-college-graduation-rates) (our is somewhat higher than that). I've routinely witnessed courses from remedial level up with 50%-60% failure rates. (In contrast, 4-year colleges only have 60% graduation rates after six years.) Many of our students cannot read or write at an elementary level, handle the simplest elementary arithmetic, have emotional/intellectual disabilities, etc. There is no threshold we could possibly set that would serve to pass all or most of these students. 

Example 1: I've had students ask whether they were guaranteed a passing grade as long as they physically attended every class session, and were incredulous when the answer was "no". (Apparently that's fairly common in some courses now.)

Example 2: A few years ago we had a university-wide remedial algebra exam for all students (e.g, at the 8th-9th grade level). I attended a central planning meeting where someone asserted something like, "Our goal was to make an exam that no one could possibly fail. We have not succeeded, because 50% of the students are still failing." (This was given as a positive argument for further reducing the standard of the exam.) Ultimately this was found to be an impossible endeavor, so the college has now abandoned the exam and the basic-algebra requirement entirely. 

Example 3: In light of budget and enrollment pressures, among the university's new endeavors is to more widely expand advertising and enrollment to even more severely learning-disabled and intellectually-disabled prospective students. 

The OP identifies a keenly-felt and significant problem. But granted my perspective, I might suggest that there is *no solution* to this problem. No matter where you set the threshold or cutoff, there are more (many more) students lined up further down (arbitrarily further down) the skill and intellectual ladder hoping for the same judgement. It's a cycle that has no hypothetical end.