Bitcoin is a technology and technology does not last forever. [There must be a new technology better than Bitcoin, so evolved human]
Bitcoin is a software protocol. It can be updated/upgraded throughout the years to make sure it remains relevant and that's exactly what has been happening.
Another important aspect is that in the crypto world having "better" alternatives doesn't mean they will surpass the "older" protocols. Bitcoin is still the top dog. Ethereum is firm second, where in both cases there are "better" alternatives but they fail to convert that into success because it has little meaning here. It has proven to be nearly impossible to gain ground as new project, and another thing is that the majority of the projects that do pop up nowadays are centralized.
Innovation is what makes Bitcoin stronger than other coins. The altcoins are quite stagnant and many of them don't last for much longer than their first speculative run. Once everyone turns a quick profit they dump it and look for a new opportunity.
Bitcoin has stood up to this test so far. It went on several big runs, and it's still standing on it's own. Yet another run like that seems a bit doubtful at this point, but we never know. Bitcoin continues to surprise.