Saying that "lots of people" are replacing their laptops/desktops with iPads is pure BS. Show us the actual data that people are doing this.
Secondly, so what if someone is using a 5 or 6 year old computer like a laptop. That's a meaningless statement and you've missed the point.
By design laptops and desktops are best at productivity. That's the hardware and software. Point and click input (precise), bitmap screen, physical keyboard, large screen, powerful CPU/GPU, and software designed for such hardware. By design, smartphones and tablets are best at consumption. That's multi-touch input (imprecise), small screens, low power hardware, sans physical keyboard, stand, etc.
I design software for a living. For Mac OS X, Windows 8, and iOS (iPhone/iPad). My main piece of software I designed is a desktop publishing program. We're currently working on prototype designs for a browser based version and one that could run on a tablet (not shipping this latter).
There are so many problems with productivity on tablets and smartphones as discussed that it's not worth the effort for us or customers looking to be productive on a tablet or smartphone. Not for iOS anyway. For Windows 8 tablets and smartphones? Yes, it's much more tenable. Now that is an OS along with something like the Surface that goes way further along the line when it comes to productivity compared to the iPad.
Yet the Surface isn't as good a tablet as the iPad in terms of just holding it and using it as a tablet.
Without any file systems, no built-in stand, no physical keyboard, and low power hardware with limited software and imprecise input, devices like the iPad won't replace laptops and desktops for anyone needing to be productive on a computer. And it was never designed to do this either. If it was, by now, Apple's desktops and laptops should themselves be out of business. But they're not... and offices around the world would have just iPads sitting on the desks. But they don't, not that I've ever seen and I've been travelling for business and in and out of many offices over the past year.
What devices like the iPad and iPhone have done is supplemented and perhaps, in some cases, replaced certain specific tasks that people do on computers. For example, surfing the Web or reading PDFs. But their uses are limited in terms of what they're really good at. They fall flat on productivity (writing reports, managing files, desktop publishing, graphic design, research, etc.).
And here I am tonight trying to watch a movie on Netflix with the iPad Air. I got frustrated and switched to my laptop. Why? Because there's no bloody stand on the iPad! I got sick of it sliding around and having to hold it in place...