People state stuff like this regularly, but it's not correct. The vastly overwhelming majority of people still have and use a PC. The only thing that's really changed is that a 10 year old PC is good enough for just about every normal task short of high end gaming or a handful of rare tasks. So sales slowed dramatically.

But even with dramatically slower sales, that's around ~300 million sold per year. By contrast, the iPad is selling about 60 million units a year. In the US at least, the number of people that replaced their computer with a phone is negligibly low, and it's largely made up of extremely low income families.

Incidentally, phones are also headed for the exact same fate as PCs. The one saving grace they might have is that lion batteries die over time which is why Apple is quite adversarial with regards to users changing their batteries, but they are already losing that fight on multiple fronts.

Or other tablets. There's a lot of good ones out there, and they are popular enough.

Handspring and Palm sold millions of devices, and so did HP/Compaq, while Apple couldn't get traction earlier with the Newton. There's no real technical reason why the Newton was destined to fail, just as the other devices ascended and then fizzled out. There are network effects at work, society has to be receptive, and the price has to be acceptable.

Sure, but is it fair to call something an unsuccessful product given an arbitrary caveat? With the caveat of "brushing my teeth", iPads and smartphones are a complete disaster.

I don't know the definition of a "successful product", but "selling billions of them every year" doesn't exactly sound like a failure to me.