In America, I don't think it'll ever be an issue, especially with concerns about the current unemployment being a paradigm shift of systemic unemployment finally being brought about via the automation of lower skilled jobs, as well as the shift towards the wealthier(corporate/stockholder) receiving all of the gains of increased production.
But, imagining that that isn't the case, and that more of America starts to be financially literate and reduce consumption(bonus points for the environment), as well as possibly ER'ing, I'd imagine we'd at that point have a cultural conversation/shift away from the 40 hour work week. Why should we all have to work 40 hours as the baseline? What if everyone in America, with their decreased consumption, their eyes towards optimization and minimalism, looked at work through that lens and thought: Hey, work sucks, we could reduce our hours collectively, have less problems with employment, etc. etc.
One thing I will say about the concern about more people ER'ing, is that over the near term, more people will leave the work force as the babyboomer generation retires. During this time, our quest for higher and higher GDP won't be necessary. We could have stagnant -real- GDP, or even slightly negative, and the economy would still be chugging along fine, and unemployment would probably be dropping as well.
But like I said at the beginning, I think automation, robotics, and computers will increasingly displace people(I know it's been theorized since the 1800's, but it will only continue to be proven wrong until one time we do find ourselves on the other side of that trend), and that is more, the conversation we want to have. Not one on what happens when people choose not to work, but what happens when most people -can't- work, -can't- compete with robotic labor.