The cost/benefit formula you're proposing works out most in SSTO's favor if you literally have just one one launch vehicle, but you're using it constantly. If y

The cost/benefit formula you're proposing works out most in SSTO's favor if you literally have just one one launch vehicle, but you're using it constantly. If you're allowed to have more than one launch vehicle, though, and can interleave their usage, then that gives you a lot more options for how to maintain throughput while controlling costs. Oftentimes the optimal solution to a multivariate problem ends up looking decidedly sub-optimal when you focus on just one variable at a time. Also, YAGNI. Right now, it's not certain that we will ever see a time when trips to space are as commonplace as commercial flights. It's far enough off that, even if we take for the sake of argument that it does eventually happen, there's no particular reason to believe that the vehicles we use to do it will be chemical rockets.