I wouldn't say it's immortal but it will be the most long lived. Some new better unimagined technology may come along some day, quantum money that doesn't even need a ledger for instance......don't ask my what or how, the point it something better always comes along eventually otherwise we'd still be trading with shells and stones.
I don't see any other cryptocurrency replacing Bitcoin though, they have their own use cases and roads to walk.
Bitcoin is just a product of technological development and technology is still developing, even tends to develop at a faster rate. So I also agree that bitcoin is not immortal and there will come a time when something better will be created to gradually replace it, just like what it is doing with fiat money, gold...other traditional assets. But one thing we can be sure of is that it won't happen anytime soon and the process will be gradual, not sudden. So if we can live until then, we will have enough time to adapt.
Looking at the overall development of the cryptocurrency industry, it is true that no altcoin can replace bitcoin in the near future but anything is possible.
None of us can know what awaits us in 10 years, and what will happen in 100 years can only be guessed. The Bitcoin network will work as long as there will be people interested in this technology who support the network. Technology does not stand still, so I have no doubt that future networks will work thousands of times faster than today's networks. Whether the Bitcoin network will be needed then can only be guessed. But if we take into account that this network is supported by the governments of some leading countries, we can say that they are interested in this technology.