Could you perhaps give some plausible argument for exponential discounting, or some record of anyone who has seriously considered applying it universally? I appear to discount approximately exponentially in the near term, but really its a reflection of my uncertainty about the future. I value future humans about as much as present humans, I just doubt my ability to understand my influence on them (in most but not all cases).

Even if you accept exponential discounting, your physical arguments seem pretty weak. How confident are you that faster than light travel is impossible, or that there is a fundamental bound on entropy / unit space? Are you 95% confident? 99% confident? How confident are you that reality looks anything like you think it does, at the bottom of the abstraction stack? Minimally, you should care about the future on the off chance you are wrong. You can still get 1/100 the utility of a civilization growing much faster than light, if you are only 99% sure it is impossible.