I don’t think it’s impossible that at some point humans might only speak one language, but I think it is unlikely. Language changes all the time, which means that if there are groups of humans who are separated from each other (e.g. in different parts of the world), even if they start out speaking the same language, that language can change so much that we might think of them as two different languages. This sort of change happens for many reasons: pressures from the environment to talk about the things that are relevant to that environment and that group of people, or as markers of belonging to different groups. In the modern world, we are connected to people all over the globe, and English has spread around the world widely – but still, most people in the world do not speak English and those that do still often have another native language as well. So even when there is a language as widely spoken as English, it is used for a purpose, such as to speak to people from different cultures, rather than as a real replacement for other people’s native languages. I think it would take a big change in how we live, such as how many humans there are and where we live in relation to each other, for us to speak only one language.
I should also mention that having linguistic diversity is mostly thought to be good thing! It tells us things about other cultures, and also how our brains work, i.e. that humans can learn languages that look so different.