I agree that such optimistic scenarios are possible; my gloomy comments aren't meant to prophesy certain doom, but rather to shake what I perceive as an unwarrantably high level of optimism and lack of consideration for certain ugly but nevertheless real possibilities.

Still, one problem I think is particularly underestimated in discussions of this sort is how badly the law of unintended consequences can bite whenever it comes to the practical outcomes of large-scale social changes and interventions. This could be especially relevant in future scenarios where the consequences of the disappearing demand for human labor are remedied with handouts and redistribution. Even if we assume that such programs will be successfully embarked upon (which is by no means certain), it is a non-trivial question what other conditions will have to be satisfied for the results to be pretty, given the existing experiences with somewhat analogous situations.