I agree that an Android device (or whatever competitor device you prefer) is not a replacement for your dev pc, but I'd wager that the vast majority of people who purchased PC before the explosion of smart phones did not use their PC like you or I. They used it for browsing the web, facebook, email, youtube, etc.
I'll also wager that the smart phones have most certainly replaced the PC for the vast majority of these users. Most PC owners used them to consume not create, and for that the smart phones and tablets are now the standard for these people.
Are they more productive? Probably not. Are they cheaper, easier to administer, better battery life (compared to laptops), more convenient? I'd say so... Not to mention, they are oh so more trendy!
So without being all technical, I'd be happy to claim that Android style devices have in fact already replaced the PC for the common person (who I'm sure are the vast majority of device purchasers).