I think the iPad is a "game changer". Laptops and netbooks are derivatives of the desktop PC. You have your screen, keyboard and pointing device (mouse, trackpad, or thumb-nub). Tablet PC's introduced some kind of direct manipulation on the screen, but the design is still tacked on to the desktop model. All these are just desktop PC's with a desktop OS on smaller, portable hardware.
The iPad eschews the desktop OS and desktop input methods. You might be doing the same stuff on an iPad that you would on a laptop/netbook, but how you do it is different and IMHO "game changing" for the industry.
Yes, the iPad is a "game changer" for some of the reasons you cited.
But it goes a bit further....
Since the advent of affordable personal computers, we've done things with it because that is all we had available. Whether or not it was the best device for the task was secondary. People who needed to move their computers from place to place bought cases and carts to facilitate transporting their systems to other locations. They had no choice.
When affordable laptop/luggable computers arrived, it was a "game changer". It wasn't a game changer for everyone who used computers at that time, but it DID change the game for those who needed to use their computers in more than one physical location. It wasn't perfect, but it was the best option available at the time.
Some of the people who bought notebooks wanted a mobile device that could surf the web, check email, and create documents. They bought a notebook because that was the best tool
available for the job.
Enter netbooks. The original promise and premise of the netbook was a reduced function, reduced footprint, reduced complexity mobile computer primarily for surfing the web, checking email, and light document creation. This was a game changer for the surf/email/doc-on-the-go crowd. They no longer needed to lug around a cumbersome notebook (relatively speaking), the netbook still wasn't perfect, but it served that role better than a notebook.
On the other end of the gadget spectrum, people used smartphones for similar purposes. They weren't perfect either, but they served that role better than notebooks for those people.
Now on to the iPad. I won't rehash all that it can do and how it overlaps a variety of market segments. But the iPad is a game changer, maybe more than those other technologies, because it satisfies the needs of some ebook readers better than dedicated ebook devices. It satisfies the needs of some netbook users better than the netbook did. It satisfies the needs of some smartphone users better than the smartphone did.
Just because other devices can do what the iPad can do doesn't make the iPad unnecessary. Just like the pre-existence of notebooks didn't make netbooks unnecessary.
For many people, myself included, netbooks were a compromise solution to mobile computing needs. I bought a netbook at the time because that was the best option
available. (I can't buy something that isn't sold) But the iPad better suits my needs.
But for others, netbooks are still a better fit for their needs than an iPad.
Every person who claims that the iPad is a toy is like that old guy on the block who refused to buy a car with power windows, door locks, and air conditioning....because they were useless gadgets.