I don't disagree with you; Nintendo's plummeting sales speak for themselves. The iDevices are marketed as devices that are able to do just about anything, but realistically it is the third parties enabling them to do so.
The way I see it, Nintendo likes to market themselves as a specialised company, and it's starting to prove their downfall. I don't believe Nintendo would consider opening up their consoles to everybody. I know a lot of hackers and developer communities just hack their devices to run homebrew, but, being a console with specialised games, it would inevitably end up with the system being unlockable and pirated games to be executed, unless they made restrictions on the way Developer games are run or something.
Nintendo, like any software company, are fearful of piracy, and I'd say that the DS was a good wake-up call to them how easy it was to circumvent the AP on the system. Currently 3DS games have been ROM-dumped, but nothing can run them, and they wouldn't want to release anything that could jeopardise that.
For the record, I know homebrewers != pirates, but where there is a means or a possibility to get things for free, people will attempt to do so.