Cryptos like bitcoin and ethereum have demonstrated that once they are created and the network is operational on a large scale, from that point on they fail to innovate fast enough. Long-term, older coins like bitcoin and ethereum are more likely to fail, and they are likely to be replaced by upstart coins. That said, which ones? There are a lot. Bitcoin for example still has the 10 minute block times, even if it ran as fast as it could. There are other coins with 30 second block times. There are coins that are basically instant just like Steem.

A person could never really pay at a cash register with bitcoin like we do with credit card systems, nor could we use Ethereum really, their bitcoin system just can't handle it, unless we just route it all through a 3rd party service that holds all the funds and the keys... and if they ever get hacked all your money is gone. Otherwise, bitcoin isn't fast enough without radically changing the software.

I'm not sure what trick the prepaid cards use, but likely you send the funds in advance of spending, and they turn your crypto into fiat on their end to mitigate risks, and then deduct funds from your fiat prepaid balance.