1) True Android is the one that runs Android apps - difference in glow or purity not counted. They run, it's Android.
2) You cannot force your definitions of Android on people to get them to buy "the one true Android". People buy what they like. They like the S3 better than the Nexus. Talk about different priorities. S3 is starting to get the Jelly Bean update too.
3) There is something called emphasis in good designs. Android was designed to let people have choice. It was designed such that Samsung can ship TouchWiz and users can effortlessly install $ANOTHER_LAUNCHER to get the experience they want on the same phone.
4) Developers have dealt with complexity arising from diverse devices before. It is not a big deal. If you are crying about testing effort you aren't doing it right. If you care about reaching to 50% and growing number of Android users you are just going to have to do it right and invest in testing your app on most phones by releasing timed betas or buying your own dozen of phones. Remember it's been done before, done right and done by god knows how many developers that have good apps in the Play Store.
5) You can't crib about higher maintenance costs if you buy a big house. You similarly can't crib about differences in Android skins if you bought into a diverse ecosystem. Good news is you have a choice - care about purity, updates etc. - get the Nexus. Care about bleeding edge hardware, 2GB RAM, quad core CPU - get the S3. Care about iPhone-esque build quality - there is One X. As long as there is something I like there is no problem. If I don't like something in iPhone land - I have no recourse. That's their emphasis in design - less choice.
Android is working as designed - sure there are those upgrade issues that can be made better - but for the most part it is not the normal users who are complaining. As long as they can get things done with their Gingerbread phone they are happy. The next one down the line will get them a Jelly Bean phone that can still run the stuff they bought.