I think it inevitable that sometime in the future we will move to some form of completely electronic currency. Whether it is centralized or decentralized.. Paper money is expensive and difficult for governments to track!
I think that BTC and LTC can be used as a saving currency. But is there zero chances that any of the current crypto will be adopted as a mass spending currency.
And yes, something better will replace them. I know that are many zelots that think in their delusions that people will get used to to work with many decimal point currency, or tonal system and other craps, but this will not happen. Also the block chain and the transaction time is a big issue. You will need mobile storage to carry your crypto in order to spend. And mobile storage is expensive. And in the end who on earth will want to carry a wallet that is 10 times more valuable than the money carried?
Regarding transaction time, is there any sane person who really think that there will be any store to accept crypto and wait at least 10 minutes for every customer to approve the transaction? This store will be out of business in a few weeks.