My fear is that that adoption will be held back and hence the rate of network propagation will not be an issue for a very long time.

What kind of adoption is held back with 10min block time versus 2.5 minute block time?

everyday in-shop purchases shops can accept non-confirmed transactions -- the risk when handled properly is minimal compared to things like counterfeit cash or credit back claims
web shops can always wait for confirmations if they want -- 10 minutes will not affect a two-day delivery
Waiting for a confirmation on a major purchase you only make every few years (say, a car or a house) is not a big deal

I can't think of a use case where waiting 2.5 minutes is acceptable but ten minutes isn't. What did you have in mind?