_Disclaimer: I'm nowhere close to being a professional in the field. If you are or simply know more than I do, please correct me._ But interstellar travel doesn't have to be based on higher speed, just like we didn't expand our energy production by mining and burning coal faster. We found ways that produce much, much more energy in the same time without needing to keep improving our fuel production. Why wouldn't we be able to find ways to travel much, much further in the same time without improving our speed? We know spacetime can theoretically be manipulated: anything with a mass does to some extent. Why wouldn't we be able to harness that someday?