Hmm.. this actually looks like developer is going to start making less money the moment they release the first feature pack. To make things fair to users [1] you have to start selling base version for less money at some point to ensure that the full price of the application stays the same, but if you do that you have no guarantees that people will buy "feature packs" - actually it's guaranteed that not everyone will buy those so you will get less money than you would if you could just stop selling version 1.0 and start selling version 2.0 for the same price (and provide an upgrade path for existing users)

EDIT: clarified a bit