It would be sucky if Apple stopped updating the iPad 3's software now that the new one is released. But I don't think they would - Apple has arguably supported older devices way better than some other manufacturers. My guess is that you'd get two more years of software updates (that's pretty much what the iPad 1 got).
Also, think about the poor early adopter of the iPad - flew mine in from Hong Kong back in 2010. Could only use it two years and then Apple said it won't support IOS 6 for it... :'( Only, fine, I can still use it and it still works great and does anything I want it to do, but Apple totally screw me over by bringing out new products and new software.
</sarcasm>
Also, think about the poor early adopter of the iPad - flew mine in from Hong Kong back in 2010. Could only use it two years and then Apple said it won't support IOS 6 for it... :'( Only, fine, I can still use it and it still works great and does anything I want it to do, but Apple totally screw me over by bringing out new products and new software.
</sarcasm>