I don't think BTC will be replaced in short term. Medium term, ETH would be a good pick. ETH has decent coverage and almost every place that sell BTC for fiat also sells ETH. localbitcoins is probably the only exception. There are other alt-coins rising and gaining recognition fast. In the long run BTC would be like the original iPhone which everybody remembers but used by pretty much no one at all.

I agree that beating the double spend issue was one of the biggest of not the biggest achievement of Bitcoin.