>And it may well help launch it, but that’s where its relevance ends.

You can expand in parallel. If Starship works you can launch 100 spacecraft instead of 1, or in other words, you can expand your system by a factor of 100 "in parallel" and hence increase whatever you get from that small payload margin by a factor of 100.

Besides that there are other approaches like laser array + photon sail which more directly benefit from mass to LEO.