Eh, I don't think so. Netbooks will likely outsell tablet and slate devices for a few years here but major companies are getting behind this "new, magical, and revolutionary" platform. Apple, HP, Dell, and even Asus are looking at the future of their slates. I won't think that slate/tablet sales will take away from netbooks though as the two market segments are for two completely different purposes. It would be like me saying that video capable portable players will take away from laptop sales as they both play videos or that cellphones will push digital cameras out of the market because every cellphone has a built-in camera.
Two different markets with two different devices. If anything, I see there being two major portable computing markets: slates and notebooks. I think that netbooks will be slowly absorbed into notebooks just as soon as the hardware is available. In other words, we will no longer need dinky single-core Atom processors in order to have a computer in a 10" package with 10 hours of battery life for $350. Ultra portable notebooks keep encroaching on territories owned by netbooks and that will continue until both markets are one in the same. Then people will have a few options for portable computing: a less capable but much more portable slate, an ultra portable notebook (11.6"-13"), a mid-sized notebook (13"-14"), and a desktop replacement (15"+). Our traditional netbooks will no longer exist once someone (Intel, AMD, or even ARM) is able to come out with at least a dual-core processor that performs better than the single-core Atom line, packs that in a device with a small screen, and gets 10+ hours out of it on a single charge all for $350. That is when netbooks will not exist as we know them and will have been absorbed into the main notebook category.
Slates will continue to occupy that middle ground between smartphone and notebook. They will be much more portable than netbooks (even the iPad is a lot more portable than my 10" HP netbook) while focusing processing power on running specific programs (or apps, they are one in the same) designed for the portable operating system they are running.
The main issue is that consumers want something new. Back in the day, notebooks used to be new and they were expensive. They saturated the market so sales slowed down. Consumers needed something new and a little more portable so netbooks came out. Given their price points, they were able to flood the market and now many people have one. So the netbook market is becoming saturated and sales have slowed down. Now consumers want something new and that is where slates comes in. They are a little more expensive than netbooks but are a lot more portable by coming in at 1.5 lb or less and taking up the same amount of space as an issue of Time magazine. Come back a few years from now and slates will have saturated the market and consumers will move onto something else.