This whole thread feels wrong somehow.

There's this underlying assumption that success in terms of gaining market shares is something that "Linux" (who-/whatever that is) certainly wants and that there's some details that prevents it from happening. (Again, I read between the lines: If only we could iron out those details, then...)

The assumption as such is wrong. There's a fight for market shares among commercial products, and ATM it's MS and Apple who are the main players.

The reason why GNU/Linux is there in the first place is not to conquer the world and beat MS once and for all. (Who would benefit from this, anyway?)
GNU/Linux is there for those who need/want it the way it is: free, open, a playground for experiments, a great learning aid and the unique chance to individually put together your components (from all the diversity out there) in a way so that they best suit your needs.

This is a niche thing and always will be. 97,962% of the computer users don't want to learn about their systems or customize their boxes the way they could with GNU/Linux. They are far better off with Windows or OSX. And there's nothing wrong with that. There's no reason to convert them to GNU/Linux.

It's a bit like healthy food:
You could argue that people would eat a lot more vegetable and less burgers and french fries if it only carrots tasted a bit more like fries and broccoli didn't look and taste like broccoli. Then you could suggest they'd sell vegetable with artificial colors&taste and prepare them with a lot of fat so that the new taste really is strong. What do we have then? Artificial carrots without vitamins but with hundreds of calories and ominous chemical ingredients instead. - Would those who wanted more healthy food be happy now that people eat those carrots? No.

So, could we streamline GNU/Linux to make it mass market compatible?
Yes. But we'd take away everything it's there for. The diversity, the experiments, the learning (and failing), the openness. And in the end we'd have a system that maybe could compete with windows, but... I would choose Windows then. It's the original, and the castrated mainstream-GNU/Linux would be a copy.

(And before somebody says so: Of course the existing diversity lets you put together individual systems that come very close to a mainstream, consumer-friendly OS. Ubuntu does a good job here, and, yes, ITOS isn't all that bad, either. But still they heavily rely on core components not under their control, so I guess it will never be 100%.)