Thanks everyone,
Helpful comments. My needs are simple, the main one being that I want to be able to import the Mathematica expressions (lists, associations, etc.) exactly as I have exported them, so that I can continue analyzing the data. Speed is not a big deal because, while I am analyzing over 100,000 files, the stuff that I will be exporting from each is small. Compatibility with future releases of Mathematica is important, of course; so if Wolfram is thinking of dropping WDX, then maybe I should learn more about MX.
Gregory