A qualifying paper must be in a field that people are still thinking and talking about in 2050. Will graduate programs still be using MWG then?
maybe. After all it has lasted nearly 30 years already.
Recent economic theory that will stand the test of time
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Problem will be that it eventually has to be updated for "recent developments." New authors will repackage a substantial portion of the traditional material with a selection of updates that reflect recent advances, and write new chapters to focus on the newer developments. Take a look at Rader Theory of Microeconomics 1972 or Malinvaud Lectures on Microeconomic Theory 1972. One could use a lot of that to teach a course today - of course, no one does that.