Knowledge isn’t just about learning what’s possible, but also about learning the limitations of what we’re reasonably sure are impossible. The more we know about the world, the more weight a “yeah, we’re pretty sure that’s impossible” holds. And yeah, as it is, FTL travel is generally agreed upon to be literally impossible since it breaks causality. Not that it even matters, since an object with significant mass getting up to any meaningful fraction of light speed will be a big enough challenge in and of itself.
In the past, people easily believed in tales of dragons and other monsters. Today, we know that hexapod vertebrates are evolutionarily impossible, so we can confidently say that dragons and other four-legged, winged creatures will never be discovered in the future. Knowledge means knowledge of possibilities, but also knowledge of boundaries and limitations. Lightspeed certainly seems to be one of those limitations that will most likely never be broken.