Well, that's false.
First, the actual mean work-week is not that high, once you take unemployment into account: some people are working their ass off, but many others either have precarious part-time jobs (fewer hours, but often no reliable schedule), or don't work at all. Reducing the official "full time" work-week would merely force the capitalists to re-allocate work a bit more equally. This will mean less unemployment, and less expensive unemployment insurance —which plays a big role in compensating for the loss of gross revenue.
Second, we could consider allocating less money to capitalists, and more to workers. The ratio used to be 60/40 in the 70s. Now it is more like 70/30, if not worse. Taking that ratio back can easily compensate for 20% cut in our working time.
Of course, such a change is impossible to initiate through individual action alone. You did capture the current conundrum of the isolated proletarian: work one's ass off, or don't earn enough. But there is a third alternative: rise up and change this rotten world.
Not easy. But as history has proven to us time and again, the world can change.