If all companies fire 50% of their engineers, how will anybody find similar new jobs? In an ideal world software productivity doubling WOULD be a huge boon for the economy IF companies used the increased productivity of their engineers as a way to manage tech debt, R&D and other issues that were put in the backlog because historically there were no resources for this. In reality all companies look at increased productivity as a source for layoffs which does not translate in higher output but the same output done by less people. Which is a net negative because now you have 50% of all engineers without a job and no discernible increase in quality of deliverables.
If software engineer productivity basically doubled as is being claimed in this thread, I think you'd see companies scrambling to lay off everyone else in an effort to hire even more software engineers. They'd be by far the most valuable and productive employees at every tech company and you'd be foolish not to have as many as you can. I'm being a bit facetious but throughout history when a resource or profession takes a dramatic leap in efficiency, the demand for that thing rather than decreasing as is predicted here, only increases since it has become far more valuable & effective.