The Shinigami started an all out war with the Quincy 1000 years ago (thus the 1000 year war - arc). The massacre happened 200 years ago and seemingly ended the dispute (all mayor Quincy were probably dead at that point (from previous battles or old age), thus it was a one sided battle, a massacre). Ishin lost his powers over 20 years ago and Ichigo is 18 (?) years old... then again Bach is over 1000 years old like it is nothing .
Nah, there is still Squad Zero, which is way more trustworthy than Aizen, plus Ishin and perhaps the off-panel swordsman.
They reduced their fighting power, greatly. Eliminated a few thousands of Shinigami, including seated members, VC and Captains, especially the general. They obtained additional power by stealing their bankai. They prepared a stage for their main course, but unfortunately they were unable to obtain any of the deserts (at least for now).
Now I have to wonder what "The Last Quincy" title really means. Perhaps it's a person that is chosen to pass on the Quincy abilities in case all other Quincy are wiped out.
FGT... a fusion between Zan and Person... heh, so that's why Aizen wasn't able to do it right away, because he is not a Quincy. It seems that this power might be the true purpose of Quincy stealing bankais. If you have noticed, thanks to Quincy Complete Form's ability to absorb spirit particles to the max, Opie was able to fuse together with a Hollow/Arrancar being. Perhaps, the Quincy's aim is the FGT, that can only be obtained by Quincy, since it is the fusion between Zan and a Person. Ishin called it FGT, but it actually might have been the Quincy Final Form and the mugetsu blade is actually an arrow :O... hmmmm ?