> Salaries across industries in the US have remained flat since the 1970s

What do you mean? The real (meaning adjusted for inflation) hourly wage in the US has increased by around 20% since 1970.

What has changed since the 1970s is that wages are no longer coupled to productivity. Perhaps that is what you are thinking of? But that should be an obvious truism for anyone in tech. We create the very things that cause that to be the case!