I assumed that Benjamin was a decimated tribe after the Concubine at Gibeah incident and decided to build a MATLAB simulator to see whether with intermarriage with Judah, Benjamin would disappear (or find an equilibrium). When I looked for a reference point from which to start that has a figure for both Judah and Benjamin, I find Chronicles 2:14:7 in which Judah and Benjamin fields armies of 300K and 280K respectively against the Ethiopians.
Unless one says that Judah had a big draft dodging problem, the tribes appear to be approximately equal in size.
The only possible Biblical explanation is that when Sennacherib besieged Jerusalem, he killed or carried off the Benjaminites, but this is not written.
What is the official position on why we assume people aren't Benjaminites?