> some linguists have hypothesized that analytic languages are easier for foreign speakers to learn as adults, and that this causes languages with lots of geographic spread and inter-language contact to become more and more analytic.

That's an interesting theory, and certainly fits with Mandarin (aka "Common Speech"); but how well does it fit with the prevalence of Greek in the ancient world, Latin during the Middle Ages, and Arabic in the Middle East and North Africa now? Those are all towards the "synthetic" scale, aren't they? Did/have those languages drift/ed to become more "analytic"?