[icegreentea]

Staging isn't free. While true that the second stage vehicle will be simpler, the combined second and first stage together could be more complicated/heavier than the Skylon.

Noticeably, the point of the Skylon and SABRE is that it's possible to use a common engine core in airbreathing and pure rocket mode, thus potentially generating weight savings.

I'm sure someone sat down and did the calculations. I have this feeling that a lot of the times when you actually sit down and work out the math for a range of assumptions, you generally come down to 'eh, maybe A, maybe B, so may as well try A'.