Why wouldn't tablets be more like, say, the iPod than the iPhone? The phone market is significantly warped by carriers. When people's contract are up, they walk into their carrier store and ask the salesperson what phone to buy. Articles about the Lumia 900 launch indicate AT&T is planning to push it as their primary high-end phone[1]. In this regard Apple does not control their sales channel as they do with all of their other products. This is significant, as recent data show that the vast majority of iPhone buyers purchased their device at the carrier's store[2].
The only real vulnerability Apple has in tablets right now is price, but I think the chances they'll leave a price umbrella where people can continue to swoop in at $199 and undercut the iPad is very small. I expect their pricing to play out like the iPod, where eventually there was a model at every price point from $50 to like $400 at $50 increments. They've also done this with the iPhone, where what started out as either a $500 or $600 phone is now sold for $300, $200, $100, or $0.
If the quality of competing tablets does not improve significantly, there is nothing at all inevitable about the tablet market playing out like the phone market.
1: http://techcrunch.com/2012/04/09/att-going-big-with-lumia-90...
2: http://www.washingtonpost.com/business/technology/apple-only...