Has anyone entertained the thought that despite all of the short comings that are so obvious to you guys, maybe it's just that the larger GW buying community doesn't actually like the aliens ?
Space Marines and their ilk have a huge appeal that goes far beyond any rubbish about their rules or support or any of that. How many new buyers of armies would even have the slightest notion of those things ?
Space Marines are popular as the are easily identifiable as human and the good guys, new players can relate to them far better than any of the other offerings.
I can see your points. Space Marines are human (sort of, if you discount the insane amount of genetic modification and implantation needed to make a one) and the good guys (again, sort of. Depends on the chapter). But the question then becomes what is the appeal to gamers?
If the appeal is "human", then why not make a go of pushing the Imperium as a whole, with emphasis on IG since they are the backbone of almost all human military operations.
If the option is "the good guys", then then they could push Eldar & Tau, who are usually described as good aliens in the fluff.
IMHO, GW has always pushed space marines way harder than any other army, which is why we have what we have today where the majority of players use (or at least own) space marines. If they wanted to, they could put a fraction of that creativity/time/effort into fluffing up/pushing the xenos races and I think we would see way more variety and many more xenos players.