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.