There is a big difference between the tech industry in general (and it may very well be the case that tech stocks could indeed be overvalued -- I haven't done any analysis of the historical data over the past few months so it is difficult to say whether that is the case) and data science in particular.
Many of the companies and organizations that are currently offering data science jobs are those outside of the tech industry (in fact, in many different industries -- think financial firms such as banks and insurance companies, market research firms, consulting firms, retail chains, non-profits, even manufacturing firms), simply because advances in computing have reduced the cost of collecting data on consumers/customers/stakeholders in these industries, and there is increased value in understanding the data to make better decisions. I could be wrong about this, but I don't foresee the demand for data science positions changing any time soon.
With respect to any change in the rates of foreign PhDs returning to their own countries, this would obviously depend on the economic conditions of their home countries. As of this moment, much of the economic growth has been led in non-Western countries such as China, India, and numerous Southeast Asian countries like Indonesia (which incidentally are also countries where many of the foreign PhDs in the STEM fields like physics originate). Until recently, many of the PhD graduates would have tried to stay in the country where they earned their PhD (e.g. Canada, US, western Europe, Australia, etc.) but I have heard at least anecdotally that there are an increasing number of these PhDs graduates that have returned to their own countries due to increased opportunities.