The thing that changed was perception. People no longer believe loyalty to employer is worthwhile, just as they no longer believe hard work is what gets people rich.
Post-WW2, America had a lock on global manufacturing and was like last man standing in a burned down world.
It was/is an illusion to think that could be a permanent state.
The pre-war employment situation in America looked nothing like post-war. Your race to the bottom narrative is probably better framed as reversion to a multi-polar world with bonus features of higher global prosperity and capability, lower barriers to access foreign markets (whether laborers or consumers), and mature logistics infrastructure. In short - more people than ever want YOUR job, and to live in YOUR house, and have YOUR safety net, such as it is, it’s not just some focus on quarterly reports.