Not so sure on this most popular version bit
I'm not so sure about this popular version bit "AIX 5.3, which was probably the most popular release of AIX in the history of IBM's Unix systems", maybe by volume. But as a percentage of the hardware out there, apart from 1.0 being 100% obviously on the old RT6150 style platforms I would say that version 3.x was certianly the one version that outlived its sell by date on many kit and witht hat I would say as a percentage of the IBM hardware at the time that 3.x was the most popular. Yes yes things moved on and your probably right about the 5.x flavours in relation to the tin it runs on. But 3.x flavours just hang around a lot longer than most. Remember that 4.x was a bit more of a culture change and was also the stage when IBM when experimenting with drugs, erm not drugs I mean Itanium and I was looking forward to one day being able to get a nice new cheap AIX box for home. But that came,failed and passed and with that I have still to get a old power system, one day as I just like AIX but hey I'm a sadist who likes there Unix to have a NT style registry :).
It is also worth mentioning that the effort to run 5.3 on 7+ kit would not be much effort as when 7+ was being developed 5.x was probably the bleeding edge in there labs.