Gold has never been replaced as a store of value and probably never will be by another look alike product. So i dont think BTC will ever be second, but other Altcoins will be used for everyday shopping or to use Ethereum platform or Lisk platform etc. World needs more than one digital coin.
Yeah, this is how things work, bitcoin won't be replaced and if it does, the possibility is very low. Bitcoin is just gonna hanging round among other cryptocurrencies and it may not be used as much as now in the future but one thing for sure is that it won't die out, at least for a long time.
How can you ensure that Bitcoin will last forever on the top of the world? It is impossible. I think you do not even care about other cryptos work and blindly believe in Bitcoin. Ethereum, monero, dash, zcash, etc, they all have the ability to knock down Bitcoin and sweep it away on this planet. Cryptocurrency is the new field and most of the people here only know about Bitcoin. You have to wait for a several years and understand that Bitcoin is not as powerful as you think
Bitcoin doesn't have to last forever or at least it shouldn't matter to us because we're not going to leave forever. The people that have used the first coins probably didn't even know that it'll be replaced by something so complex like bitcoin. All the coins that you've mentioned surely
knock down bitcoin but apparently they don't, because their force is much weaker than bitcoins. The number of users/supporters is what keeps bitcoin as the best cryptocurrency ever. But of course all of these can be changed over time just like how the first fiat currency was replaced.