These phones will be supported for another few years. It's something Nokia have always done, apparently, although if you've only ever had contract phones, you'd never realise it.
Normally, what used to happen was the mobile operators couldn't be arsed to sign off on their "special" versions of the firmware, so the end-users never get the updates. Oddly enough, customers who are sick of their phone's problems tend to buy new ones, complete with new 12-month contracts. Win-win for the operators, lose-lose for the customers, and until recently win-lose for Nokia (win a sale due to lack of choices but lose customer loyalty)
Future phones will use WP7, but Symbian has another few years of life in the middle of the market, where WP7's minimum spec stops it being cost-effective.