You are absolutely correct there is no "solid theory" for FTL. Rather, there is "solid theory" FTL truly is impossible in the universe as we know it, because it would allow effects to happen before the events that caused them. It would allow messages to be sent back in time. The speed of light is much more than the speed of light: it is the speed of causality itself. If the sun were to suddenly disappear, the earth would continue to orbit the disappeared sun for the roughly 8 minutes that lightspeed (or causality-speed) requires for covering the earth-sun distance. Anytime any event happens anywhere in the universe, you can imagine an invisible sphere exploding outward from that event in all directions at lightspeed (causality-speed). The event cannot cause an effect anywhere in the universe until this expanding sphere hits that part of the universe. So picture one of these spheres expanding outward from you at the moment you were born. At age 80 your birth-sphere now has a radius of 80 light-years. There is a 160 light-year diameter ball in the Milky Way all the effects of your birth are limited to. It will be millions of years before anything you ever could have done could have any effect whatsoever in the Andromeda galaxy. There is a "glass half full" optimistic view of this limitation: intergalactic Thanos/Hitler types cannot exist in the universe as we know it. We are well quarantined from the evil in other galaxies (if any exists), and other galaxies are well-quarantined from our evil here (where it definitely exists).