Denominating prices in terms of 1 wei / byte will remain viable so long as people need to charge no less than 1c a Gig for bandwidth and the price of a single coin of exchange remains below 10 million dollars. I expect this will hold true for our lifetimes.
Using u32 Max as the max price will remain viable so long as prices need to be no higher than 50c / gb and a single coin of exchange has a price above 11 cents.
This is not a problem for many coins, but is something of a problem for Dogecoin which could not charge more than 25c/gb even if they wanted to.
If we increase to u64 the minimum viable coin price to charge 50c/gb reduces to 1*10^-11 or 5 ish orders of magnitude cheaper than the cheapest coin on Coinmarket cap
My recommendations are to continue with u32 for now but slate the multi currency extension to transition to denominated u64 prices which I believe will be sufficient for any reasonable coin.