Do professors have a moral duty to retire?

A few years ago, I read the Philosophy Smoker on a regular basis. In the comments threads, several job seekers complained about older professors who didn't retire. If only they finally went away, more tenure lines would become available for junior people. In a provocative essay, professor emerita Laurie Frendrich argues along similar lines. She argues that professors have a moral duty to retire. The reasons why they don't, she argues, are largely self-serving: the large income of a senior faculty member, the pleasure of teaching: "Professors approaching 70...have an ethical obligation to step back and think seriously about quitting. If they do remain on the job, they should at least openly acknowledge they’re doing it mostly for themselves."

Unlike in the US, where the mandatory retirement age of professors at 70 was lifted in 1994, European professors are still obliged to retire when they reach a given age (usually between 65 and 67). It is certainly a good thing that tenured lines eventually open again and younger academics can step in. But does that mean that older professors in the US also have a moral obligation to step down when the time comes? For one thing, many tenured positions aren't being replaced by junior tenure lines but by contingent (VAP, adjunct etc) positions. Also, pension schemes were gutted during the 2008 and following years crisis, which made it financially precarious for older professors to retire.

I agree high retirement ages are problematic, but I disagree that individual older professors have a moral duty to retire. If the total effects of scrapping the pension age for professors are negative, that should be a reason to reintroduce mandatory pension age in academia in the US, but it does not put the burden of that decision on individual professors in their late 60s or older. Let's look at some of the main arguments Frendrich offers:

1. Deciding not to retire is a mainly self-serving decision. However, so are other decisions that younger professors take, and we do not fault them for these decisions. Take a tenure-track professor who decides to go on the market again, because the location is not geographically ideal for her, in the knowledge that her tenure line will probably disappear. I think many people would argue it's not problematic to do so. The reason most of us are in academia is for personal financial and other reasons (doing a job we love and getting paid for it). This should be more widely acknowledged. The narrative that financial considerations are unimportant is a pernicious one, leading to the devaluation of our labor and further erosion of tenure. Faulting senior faculty members for not retiring for the reason it's self-serving seems ageist, since we are singling out people based on their age.
2. Older faculty members are less productive in winning grants, innovative teaching, research. The research on the relationship between productivity and age is quite complex. Different stages in the life span correspond to different kinds of creativity.  There's a swansong effect (late contributions of composers are highly valued aesthetically), people who start to produce later tend to peak later, etc. But even if we grant there is a decline, the productivity argument is dangerous. Should we hire academics who are parents and who take on a significant load of the parental load? There is increasing evidence that academics who are parents, including fathers, suffer penalties for being parents (taking leave etc), and we think that is unjust - rightly so, I think. So it is unclear why older academics would have to retire because their productivity is lower, as long as they can meet the expectations for teaching and research that they have met earlier. 
3. A vibrant faculty life requires staff of all ages, and older professors who refuse to retire upset the balance: "A healthy university consists of departments with a balanced mix of new hires (full of energy, ambition, and fresh ideas), middle-aged faculty members at the height of their productivity, and older faculty with wisdom and a deep understanding of the evolving mission of their departments and universities. Disrupt that balance, and the foundation of an institution’s strength is undermined." - I think there is something to be said for this argument, but it depends to how far faculties are genuinely upset by this. Do we have evidence for the extent to which faculty composition is altered by later retirement ages? The picture the author paints is one of Luggnagg in Swift's Gulliver's travels, where the immortals struldbrugs hang around forever without the benefits of eternal youth. Is academia (or will it be) overrun by cranky, opinionated struldbrugs? 
4. It's unfair that older people take up to a disproportionate extent cushy, well-paid tenure-line positions: "older faculty, by hogging an unfair share of the budget devoted to faculty salaries, exemplify the tragedy playing out in the larger social and economic arenas of all industrialized nations, where older members of a society, compared with younger groups, now possess a disproportionate share of a country’s wealth." - I find this an intriguing argument that bears closer consideration. In the west, we've come a long way from the time when older members of society were reliant on the goodwill of their children (if they had them) to have a dignified old age. Those people who were lucky to accumulate wealth in times of economic growth and social mobility benefit. But this systematic pattern tells us little about whether individual older people are obliged to step aside for younger colleagues. Leaving aside the fact that many tenure lines are retired when the person holding them retires, it is not clear to me why they would be obliged to do so. Should male workers ask for lower wages because of the systematic unfairness of their female colleagues being paid less? I do think that older faculty members have some moral obligation in helping their younger colleagues, in advocating for the continuation of tenure lines (to the extent this is possible?) and fighting (in their public writing, their committee work etc) against the erosion of tenure. Again, this argument says something about the worsening conditions for junior faculty members who suffer wage compression and contingent labor, but it's not clear to me how this translates into a moral obligation to make place for them.