That’s not the counter-argument that we need.
The argument that’s needed is to show that allowing newcomers to choose their rank will not skew the rank pool. IIUC.
The whole business about sandbaggers is a distraction - a second order argument, AIUI. It’s like "we have this decision (to start people at the middle rank) and we have this problem (sandbagging) and we look for solutions at the intersection.
OGS’s purpose is surely to be a nice place to play go. One of the considerations is having a stable rank pool. Once again, it’s only “as I understand it”, but this is my recollection of the primary reason for “the way things are”.