I think this makes assumptions about the demand for software engineers that seem unproven. Law salaries are bimodal because the demand for lawyers is limited, so there's only so many big law firms that can exist. In the near future, at least, it seems like demand for software engineers far outstrips supply and I think this will stay true in the long run because the market for software engineers is just different from the market for lawyers, in that software engineers can work on a multitude of different industries (faang covers personal devices, social media, entertainment, e-commerce, search, and more).
I think the real concern is for when the rate of new startups decreases, because I view startups as basically the tech industry's way for searching for ways to increase the overall size of the pie, and startups slowing down means demand for software engineers will start to stagnate.