Smartphones offer email and messaging (they mostly also can make phone calls). They also offer all sorts of PDA functions. What more do you expect something that size to do? There's plenty of money to be made there (at least for Apple and Samsung).
I'm not as sure about Netbooks. Chromebooks have possibilities, and they haven't been out all that long, so I'm not calling them a failure. Not now, anyway.
Tablets have succeeded in the market. Apple has sold a whole lot, and you don't get Apple. They sell an experience, not hype, and no significant numbers of people (in the sales sense) get quasi-religious. The fact is that you aren't Apple's target audience, and you fail to understand what might appeal to other people. I've seen lots of people making good use of iPads. I believe Samsung has also been doing well, although I've not seen many general-purpose Android tablets. Amazon has been selling massive amounts of them, many of them fundamentally Android tablets with special links to Amazon. Nor are tablets too locked down (the walled garden has a lot of advantages for non-technical people), and they are convenient.
Fundamentally, you need to either stop making sweeping statements, or get out more and hang out with people different from you.
The market for desktops and laptops is not going away, but for a great many people they can be replaced by tablets, or even phones. I don't want to get my mother-in-law online with her desktop or laptop, but I'd love to see her using an iPad for that purpose. It'll do everything she might want to do with a computer. (Not everything you or I might want to do with a computer, but people like my mother-in-law are a pretty big market in themselves.)