I think BSP was kind of obsolete even when it was used in Source 1, but was left in because removing it would have required changing a lot of code for no real benefit. The blocky brush-based level geometry system from GoldSrc still remained, but they added a lot of higher-fidelity features on top of it. IIRC, the level editing pipeline code for things like PVS and baked lighting were still based on code from GoldSrc. Better, newer techniques existed but what Valve had wasn't broken, so why fix it?
