If your case is that BIP is insufficient to establish the conclusions Bostrom wants to establish, I'm pretty sure it does in fact suffice. If you accept both of these:

• BIP: Cr[SIM|f-sim ≥ x] ≥ x (where f-sim is over all observers in our evidential situation)
• Cr[f-sim ≥ x |V ] Cr[V|¬SIM] ≥ y_x

then we derive Cr[SIM] ≥ 1- (1-x)/y_x. x is some estimate of what f-sim might be in our world if we are not in a simulation and our current evidence is veridical, and y_x is our estimate of how likely a large f-sim is given the same assumptions; it's likely to be around f_I f_p.