The weakening of undergraduate education was not due to a thousand cuts, nor just one cause. To me, there were several (for private universities)
1. Faculty cowardice - (among some) Many faculty just do not want to argue with students about grades.
2. The quest for good teaching evaluations. I would guess that the correlation between high grades awarded and enthusiastic teacher evaluations is quite high almost everywhere.
3. Political correctness and the mania to support "self-esteem" of students.
4. Precipitous decline of honor codes almost everywhere, so that cheating is rampant.
5. The spread of "spinus dissaperanus," a viral disease that robs university leaders of whatever spine they had before becoming leaders.
6. Governing board failures, due partly to the fact that prospective board members eagerly seek the social status that comes with university board membership.
7. Growing lack of diversity in faculty philosophical views.
And many more.
What rather strikes me is how all the specific active failures listed here are pinned on faculty, while the failures of administration are given only the vaguest mention. But, then, I'm faculty, and would say that. Just like I would say with regard to (5) that precisely one of the failures of modern academia is that college presidents think of themselves as 'leaders' rather than as what they in fact are: chief coordinating facilitators for the work of education, which, it must be pointed out, is a cooperative venture carried out in actual fact mostly by faculty and students, with large assists from a fairly stable and easily identifiable group of staff -- librarians, tech support, secretaries, and the like. Very large parts of this cooperative venture are sustained by students and teachers and librarians doing things that they could perfectly well do, and would gladly do, and would often do well, even if there were no such things as college presidents. College presidents who think of themselves as leaders inevitably do stupid things, with only a tiny handful of exceptions; what a college needs is a president who has the sense to recognize that his job is to be an administrator, which can involve some leading but is often a different thing entirely.
Nonetheless I'm not sure what ground there is for some of these. I know of no teachers who are particularly worried about their student evaluations: it is in fact very difficult to do very poorly on student evaluations because (1) students are often nice, and do often like their teachers regardless of whether they like the course, and don't want to get them into trouble; amd (2) students have no clue how to evaluate most of the things they are asked to evaluate, and so tend to give noncommittal answers. And while there are, I am sure, worriers who fret over evaluations constantly, this seems to be widely recognized. I worry about my student evaluations for no more than fractions of an hour a year: the fractions of an hour spent handing them out and the fractions of an hour spent reading them. I do occasionally learn something worthwhile that affects how I adapt the course for the next term, but it is simply implausible to imagine any significant number of professors so overwrought about student evaluations that they grade with a view to them. I know too many freakish academics ever to claim that there aren't any, but it's just not likely that this is a significant factor.
'Faculty cowardice' is more plausible, but, again, it's difficult to see how it could have any large-scale effect. Just speaking on the basis of faculty I know, two far more plausible causes would be (1) the fact that teachers tend to have sympathies toward their students; and (2) the fact that teachers tend to hate grading. The combination of these seems to me to be far more likely to have an effect. Regardless, it's a relatively easy fix: separate teaching and evaluation. If we consider evaluation alone, it makes very little sense for me to be grading my own students. For one thing, it really amounts to my grading my own (perceived) effectiveness. And, for another, it largely diverts good teachers into tedious tasks -- it takes up a great deal of time that would in most cases be better spent in direct consultation with students. There are disadvantages to such a system arising in areas other evaluation, but it's noteworthy that this one move would wipe out half the supposed causes on the above list.
In the end, I think grade inflation is a myth, not because there isn't anything wrong with grading systems today (there is, but it's a different set of problems), but because claims of grade inflation buy into the idea that grades are supposed to be univocal measures of merit. They are not, they have never been, and it is not possible to make them so. They will necessarily vary from institution to institution, from instructor to instructor, and often even from course to course with one instructor. When you see an A on a transcript, you don't know what they did to get it; you don't know how it compares to what people five hundred miles away did to get an A, or even how it compares to what other people are getting in the same course or at the same institution; you often don't know the ins and outs of the college's grading policy; you don't know the aims built into the courses that led to the assignments and grading being done the way they were in that course. There are steps that can be and have been taken to reduce the confusion this causes, but they are limited in effect. What most people call grade inflation is not grade inflation at all; it is simply grade confusion, ten thousand competing grading philosophies, many of them quite reasonable, whose differences are simply ignored by arbitrary assignation of letters. I sometimes find it interesting that people don't think about the immediate implication of the claim that grades are univocal measures of merit that should not become more common: for this to be the case, it has to be impossible for teaching to be improved. If teachers can improve, we should expect high grades to become more common as teachers refine and improve their teaching skills, not stay the same, because as teachers improved more of their students would meet the standards (up to a certain saturation point). Common views of the nature of grading are shot through and through with obvious falsehoods and confusions.
And, in any case, if there were grade inflation, it would not itself be a major problem -- there are things, like the structure of the curriculum and program requirements, that can offset it. The real thing to worry about is credential inflation, which arises not from evaluation inflation but from economics, and, while it slows down on the upswing and speeds up on the downswing, it occurs regardless of whether evaluation standards are being raised or lowered.