I agree with some of what you have said but I'd like to point out that Android has very good tools for handling different screen sizes and pixel density, and since Honeycomb has had "fragments" for allowing the different phone/tablet GUIs that you are referring to. There's no doubt that fragments in particular are underutilised at the moment though and this could be attributed to the fact that Android share of the 9-10" tablet market is not really worth targetting yet. And sure, it's best if one company controls both hardware and software, assuming that that one company excels at both.That was definitely the case 5 years ago when Apple clearly had the best hardware/software combination in phones and is still true today in 9-10" tablets.
But for me personally the hardware/software experience of many Android phones (Samsung Galaxy S3, HTC One X) is superior to the iPhone so I don't believe that your theory still holds true. Sometimes having a company that makes great hardware (like Samsung or HTC) paired with a company that makes great software (Google, at least since ICS) can deliver the best product. Of course, this is based on personal opinion but like many Android users, I previously owned an iPhone so feel qualified to make an educated (personal) comparison.