if you look closely, I said that there is *** at least *** a 50% drop off with polarized light because the polarizing filters, by definition, knock out at least half of the light if they are working properly... but the actual loss would be even higher. In practice, the real-world result could look very much the same with active shutter.
The other thing that folks may take time to realize is that the alternating eye-blanking itself does not cause brightness to perceptably drop off much because your brain perceives "brightness" by the eye that sees the brighter light. Cover one of your eyes... does the room suddenly go 50% dimmer? Nope... your brain still "sees" the room just as bright even with one eye covered over/blanked out. Whatever "dimming" is perceived from alternating left/right it would be minimal and represent the least amount of perceived dimming with active 3D.
Active eyewear has transmissive loss (as you note)... that's where the real "dimming" is coming from with active eyewear. And with time technology should be able to minimize that significantly to the point where active eyewear 3D might not look noticably dimmer than the native 2D without eyewear. Time will be good to active 3D, and hopefully also with passive 3D once polarizing screens are better perfected to avoid hotspotting etc.