That's partly true but you're ignoring a few things, in my estimation.
What's the difference between a tablet with a separate keyboard and mouse peripheral, and a laptop? Ummm... I will grant that it's marginally more portable, but otherwise, is there a functional change?
But ok, let's say that portability alone makes it more likable than a laptop. How about the factor of price? Now, perhaps the trend will reverse in the near future, but I can guarantee you that as it stands, size of the machine almost always correlates positively with its power. I have a desktop and a laptop and I've used a tablet, all of which were purchased within a year of one another (actually, the desktop is a wee bit older admittedly) and the power of each is pretty much fits this correlation. The new iPads have some pretty nice capacity for graphics... for a mobile device. They have pretty good memory... for something using flash memory. They have a good processor... can you guess compared to what? Now, they could hypothetically make a tablet as powerful as my desktop, but it would be thousands of dollars. The best iPads right now cost half as much as my far better desktop did nearly 2 years ago, and I'm willing to bet they aren't half as powerful, with a small fraction of my RAM and disk space.
Will tablets get better? Yeah, definitely. Will good tablets cost less? Yep. Will better still desktops keep getting released and will they be cheaper than comparable tablets? I'd put money on it. You can already see the model in laptops vs desktops. A laptop that can compete with my desktop in terms of power is much more expensive than the desktop, and not as modular. You can't upgrade a tablet. You cant give it more RAM to keep up with new trends and needs, you can't replace the graphics card, you can't even replace the disk. This alone degrades the value of the device significantly as a gaming platform.
You say that gamers don't need the open space of current desktop and laptop OSs? Some, probably. Have you ever wanted to install a mod in Skyrim? Thousands and thousands of gamers don't just want to be able to do that, they do, and they expect every game to be able to handle that. On a closed OS you see on tablets, you can't do that stuff without significantly hacking the device. The tablet's closed system is not, currently, as viable as you are arguing for PC gaming for that reason alone.
It's not that I disdain tablets, or think it's bad that people will start using them more (they will, heck I've been thinking of getting one soon), but I don't think, in their current state, they will be competition for the value of the desktop or laptop computer for things like work and gaming, until there's some serious changes to them. I think they will become an ubiquitous part of using the internet and entertainment (and yes, this does include games - of a certain variety) but for work (especially programming, but that's my professional bias) and "hardcore" (for lack of a better term on my tongue right now) gaming, the current desktop/laptop system is hard to beat on functionality, power, price point, and modifiability. For a user friendly experience, tablets will get better, and they'll get better at those other things as well. But I'm not as willing to jump to the assumption that they will be best any time soon.