I doubt that there is any other cryptocurrency or altcoin that can supersede the bitcoin. At least not for the next 5-10 years. Of course there are many rising competitors like Ripple, Litecoin and Ethereum. But Bitcoin is historically the first and best crypto currency. Its been just a few years since the bitcoin came into existence and theres still a long way to go.
But the future no one knows and everything in the world might happen, we know if bitcoin has a limited supply and we do not know what will happen with bitcoin once the supply has been mined, and that might happen if bitcoin continues weakened and other altcoins began to soar like eth or other. I think it would be fine if there are other altcoins that replace the bitcoin place as the highest cryptocurrency. Future changes will occur and hopefully those changes will make cryptocurrency existence much better in the future.
I agree that we really don't hold of what the future could be. Once the bitcoin max cap will reach, we really don't know what will happen next. Some people are saying that since there are no more to be mined, its price will go even higher and others also believe that the transaction fees will even be higher that time as well - but who knows?