Claims like this are fun to talk about but little more than good marketing for Coinbase. (Don't get me wrong, I like Coinbase). But no one can say for sure whether bitcoin will have staying power and significance to be considered the world's reserve currency ever let alone 15 years.
What I think is a better discussion to have about this is why would we want Bitcoin to be the world's reserve currency. Existing fiat currencies are controlled and manipulated by Governments and Banks. Do you think these people are going to just let Bitcoin exist without their involvement or blessing? Do you think these people aren't going to want to have control and option to manipulate Bitcoin as they have fiat?
I think the answer is No!
If Bitcoin becomes the world's reserve currency, Bitcoin will be destroyed as we know it. I can't see a way that it wouldn't, but if you all think differently I'd love to hear it!
Many changes that improve privacy, are under development. For example, efforts to change payment API aims to prevent the sharing of addresses for payment that creates the Association between them. The change addresses, in Bitcoin Core, could be eventually implemented in other wallets. In the graphical user interface can be added to easily request payments, which prevents re-use of addresses. Work is in progress and for the development of other functions, potentially increasing privacy, such as the ability to combine transactions together casual users.