Bitcoin is the largest and most prevalent, so it has become the currency most often used to exchange to fiat. Usually the transaction fees are still mostly negligible because these are larger transactions than the faster altcoins when they're used to make purchases.
Essentially no barrier to entry, and some federation could happen.
Of course, what you end up doing is creating banks, but hey they're not regulated by the government if you are behind 5 proxies? "Hobbyist bankers" might be fun.
The ultimate frustration of bitcoin is that it can occupy the entire range of decentralization, but the community compares everything to the logical limits of "the Federal Reserve will defeat math and generate BTC" and "I don't even have to trust my own computer to do this transaction!".
It's the seller that would care about confirmations in that case. Hopefully they do something hilarious like have a bitcoin corral for people to wait in while their payments clear.
I really don't think this is as big of a problem as you are making it out to be.
So obviously there's going to need to be a credit backed middle man to guarantee your transaction so the seller doesn't get screwed by a double spend.
Eth is slightly better, but it still takes something on the order of 10m to confirm a transaction, and it doesn't look like it can get anywhere near the speed needed for a coffee purchase.