The most interesting question to me is if/when all menial tasks (and each step in complexity thereafter) can be performed by robots/AI: are there people who aren't capable of performing any other useful tasks, and if so, what to do about that?

I would propose we are somewhat in that situation already, where many employees are nearly useless in the capacities they are supposed to be working because they just aren't capable, though in possibly all cases this is solely a temporary problem due to inefficiencies in the system. It is logical to assume that problem will only get worse though. Hopefully this will lead to advancements in education, and in means of organizing and developing the abilities of those whose skills are obsolete in addressing the many problems that we still need to tackle. Essentially, mining and engineering humanity for unrealized potential.

Moving beyond McDonalds workers, think about doctors even. Many of the tasks currently performed by doctors will be automated in the next 50 years (and many tasks earlier than later). So their job description will change drastically, which hopefully means their efforts will be redirected to the areas we are currently deficient, leading to better care and health for everyone.

Efficiency and productivity advancement is of course a large net positive, as long as the benefits do not go increasingly disproportionately to a smaller subset of humanity (as has been the case the last 20 years in the US for example), and that the obsoleted workers can be redirected effectively to other tasks. Similarly, I am concerned that many human skills could be increasingly devalued as natural resources become the limiting factor, as is already happening to some extent. I don't have an elegant solution, but we really need to find a better means of balancing rewards for shrewd trading/management with maintaining a high level of parity between contribution to society and consumption of goods and services (including capital accumulation for eventual consumption). If we don't, I could certainly see a future where, despite vast increases in efficiency, wealth/power is far more concentrated in the hands of a few: natural resource owners(which China seems to realize), shrewd businessmen/politicians, engineers/scientists in frontier fields that haven't been mastered yet like AI, biotech, etc. I suppose this is really just more of the same situation we have now, but I am concerned about it being exacerbated by this trend. Hopefully political/economic/social dynamics will keep this effect in check.

Up until recently the forces of natural selection have largely applied to the human race. Since we have become more civilized and are trying to live in harmony, we attempt to respect everyone's right to exist and ensure everyone is given opportunities to thrive. It will be very interesting how policies regarding reproduction evolve as we approach the manageable population limit. Certainly the current US policy cannot persist indefinitely, which offers substantial assistance in providing the basic needs and education for an unlimited number of offspring. How will this be limited? How will it be enforced? Will we enact an equal limitation as China did? Will we adjust policy to reintroduce elements of natural selection, such as requiring some proof of genetic utility to society (perhaps simply as ability to financially provide for the child without external support). If so, will this trend in automation etc play a role in shaping the genetic evolution of the human race? I expect it will in one way or another once population growth becomes a problem, just as societal changes had for centuries before our recent socialist policies (and in many ways still does globally). Perhaps this will be a part of how the question raised by parent comment is addressed (though I am certain it wouldn't be McDonald's workers in question, it would be many many years down the road, not to mention that "McDonald's workers" comprises a very non-homogenous set).