They are willing to use euros as a medium of exchange, but only because they know they can exchange them for (to them) real currency. (Currency is relative, obviously. And just as obviously it helps that somewhere in the world there are people who use that type of money, which gives you comfort that even if you don't exchange it now, you can later.)

If this was the fate of bitcoins then it would be a failure. People may use it to send value over the internet, but no one will keep their savings in bitcoins.

To be money you have to be willing to save it. If all you do is use it as an intermediate it's not money - yet. It may eventually become money (historically intermediates often do that - especially if the conversion is difficult), but it's not money yet.

The reason it's so important for people to store the currency in savings is very simple: If no one want to keep your money long term, practically speaking there will be very little of it in circulation, which makes it hard to use.