Obviously cash will be extinct if other countries will follow, they will only have value for collectors and museums. But for the possibility of that happening I don't think so relying fully on technology for payment in the future might not be best for all of us, I mean not all people are technically inclined to use gadgets let alone make our legal tender virtually exist. It would take years before people will fully adapt to the new system as many people will still prefer the use of Fiat currency.
Not really. First thing, you should be thinking how imbalance everything would be. Underdeveloped countries who are not advanced with technologies would not be able to cope with international countries, thus causing more problems to them. Secondly, even if people do know how to use the technology, there will always be trust issues. Anything stored digitally can somehow be hacked through or cause disruptions. So people would still prefer fiat over digital money.
Banks "store" everything digitally. And physical items are subject to theft and corrosion, so your argument doesn't really hold. It's also far simpler to swipe a card or smartphone than to use real cash for transactions.
The only bottleneck is a lack of technological progress in some developing countries, but that doesn't take very long and most people already have smartphones there.
And internet is going to be beamed to the entire planet within a couple of years by Alphabet and Facebook, who want to reach the billions of unconnected people to increase their ad revenue and influence.