And now stop and ponder for moment about human inventions and development of our tech. Have you seen atomic shelters from cold war era? With big computers with processing power lesser that current watches?
Do you see how Moore's law is affecting our progress? And now apply all this to bitcoin - it might last for 10, 20, 100 years - who knows. But I am sure that in the future something else will take its place.
Even if something replaced bitcoin in the future, bitcoin's advantage would be that you could easily exchange it to that new currency. Bitcoin was and is a milestone. The new milestone should keep the advantages of bitcoin so exchanging from bitcoin to the new milestone should be easy, quick and cheap like exchanging between cryptocurrencies.