Another option is self-enforced population decline (already happening in many developed countries) - for now it's artificaly solved by immigration from not-so-well developed countries, but this will stop at some point.
While I see legitimate reasons to reduce population, that won't solve this problem. In fact it might make it worse.
While the future economy will be heavily automated, it will still be an economy. Declining demand will still do ugly things to it. It's likely that a drop in demand will do more to squeeze out human labor than machine labor, since the former is more expensive and in a declining economy everyone tries to squeeze margin.
