To answer your last question-
I don't think there will be that "one" Android phone to rule them all.
Android has never been about "one" phone even though there's the Nexus device. The Nexus is just another choice for consumers. Android has always been about choice based on an individual's specific wants and/or needs. I'm a person who appreciates that because everyone is different.
As for the UI lag on ICS, there is some but to me it isn't severe. My definition of severe lag is stuttering, very slow transitions to the app drawer, and of course freezing. None of my devices including my Ipad 2, my laptop and my phones are free from the occasional lag but it doesn't bother me because the lag isn't severe....but that's me. I'm sure you have had a different experience.
One thing I really really really agree with you on is the build quality. IMO, after the D1, Android phones have gone down hill with build quality. Manufacturers put all their time on their software and inner hardware and totally go el cheapo on the outside of the phone. I'm glad most are using gorilla glass now but that took a long time before that happened. I hope once the Processor spec wars are over, manufacturers start paying more attention to the outside of the device. (fingers crossed)