Difficulty still varies up and down to target a set number of block being generated in a set time period. However, the coinbase amounts are not following a hardcoded decreasing pattern to target a fixed final amount of issued currency, but are adjusting up and down to target a fixed exchange rate, for example pegging the currency to the dollar. (Please don't get stuck at this point on how undesirable this is, I'm not advocating for it, just trying to convince myself whether it would work.)
Aside from the problem that you mentioned with getting miners to agree on exchange rates, there is also the problem of being able to validate a block some time in the future; you'd have to have definitive historical data for the exchange rate in question going back to the beginning of the block chain to compare against. Even if you were just going to trust the longest chain implicitly when there is a single longest chain, if ever there were forking (and there would be alot of forking because everyone's rules for what a valid block would be different because invariably nobody would agree exactly on what the exchange rate was at a given time), you'd still have to have authoritative exchange rate information going back to the time of the fork.