Nobody has to buy the latest model every time it is released
and it's not that weird for a couple of updates a year to computer models, so it shouldn't be for tablets. It just means when you plonk down your money you get more recent tech for it than you might have otherwise done. That is a good thing.
In my view, the important thing is not how frequent new models appear, but how long these models are supported for updates etc,. Speaking (well, typing) as an iPad 1 owner/user, I'm not too happy how short a time Apple released updates for it (I know, gen 1, early adopter blah blah

). I don't regret my purchase, but if I see the iPad 2 being excluded from the next iOS it will put me off buying another iPad because of the sense of artificially short planned obsolescence.
I've said this before, but I think it's worth repeating - no-one should expect all the latest features or as fast hardware from older models (that should be obvious really) but maintenance/security updates should be released for years and years. It's the new features along with a secure feeling of the device having a decent lifespan that should encourage users to buy newer models, not feeling forced into it because they don't feel confident in how safe the browser or their user data is anymore...