Bearing in mind the iPad apps currently take up only circa 10% of the total iOS apps available. Apple will most likely strengthen the growth and establish stabilization until a display revision. Like the OP pointed out, a revision in display resolution in the next model will only introduce fragmentation in a market that is starting to grow, and that may backfire in the event market growth is hindered for some unforeseen circumstances.
So I guess Apple will built upon its current available technology, and offer upgrades that is in line with its traditional low cost/high profit strategy, while monitoring annual growth percentage until the point where upgrading the display becomes the only inevitable path of succession.
Right now I see no reason for that since the iPad has an almost complete monopoly on the tablet market. The demand does not justify the need.