Although GPON is shared, it is not half duplex! I'm not exactly sure why the asymmetrical version won out, perhaps due to cheaper optics back in the day?
Latest iterations (NG-PON2 and XGS-PON) are 10G symmetrical, and XGS-PON is now cheap enough that GPON doesn't make much sense. AT&T Fiber in the US has been deploying it for the last few years.
Seems like you are right! Although most are asymmetric (2488 / 1244 Mbps). I guess that even for ones with symmetric bitrates, effective upstream transfer rate would be lower than downstream transfer rate due to synchronization / higher inter-frame period needs.
Latest iterations (NG-PON2 and XGS-PON) are 10G symmetrical, and XGS-PON is now cheap enough that GPON doesn't make much sense. AT&T Fiber in the US has been deploying it for the last few years.