Part of the reason for the higher cost for the 17" is the fact that it is not a high volume model. While the internals are identical to the 15", the other components are not. Larger display, larger panes of glass, larger unibody enclosure etc. Companies can reduce costs for these kinds of components by buying them in volume, but when the volume is so low, that's hard to do.
When setting up for production of the redesigned models, it's quite possible that Apple would choose not to invest in the production facilities needed for the larger model.
If it makes up less than 2% of laptop sales for Apple, that's a really small number. Most of those people will be just fine with a 15" machine and will move there without complaint. Most of the others will begrudgingly move to 15", but will not switch platforms. A small percentage of the current 17" users may opt to switch, but I think that's an acceptable loss for Apple.
As of now, I'm expecting to see the existing 13" MacBook Pro with ODD stay available with a spec update for at least this next generation. Perhaps the 17" model may see a similar update. I don't see it getting a redesign however, and I don't see it lasting beyond this next generation if it isn't killed off altogether.