No, they won't eventually sell only cellular models - not even if it cost exactly the same to manufacture both versions (after savings on manufacturing efficiencies, fewer SKUs, etc.). However, I doubt the current $36 bill-of-materials difference could be totally eliminated by efficiencies - there's a law of diminishing returns in high-volume products; the savings you get going from, say, 1 million to 2 million units is greater than the savings you'll get for going from 2 million to 4 million.
The big thing standing in the way of eliminating wifi-only is perceived value. People expect to pay more for additional features/less for fewer features, and they hate paying for features they don't need. The first thing we'd hear in these forums is, "I don't need no stinkin' cellular, Apple's being greedy by not offering us a lower-priced wifi-only model!"
It's similar in corporate/education markets, especially all those places using iPads only on-site (factories, warehouses, retail stores, etc.). They would definitely balk at buying a cellular-capable model when it wasn't needed. School districts would come under pressure to provide cellular data in order to bridge the digital divide... Effectively, it gives the other manufacturers a unique selling point, "Apple's unnecessarily over-priced. We'll be happy to sell you a wifi-only model for $100 less."
The price gap between cellular and wifi-only models may narrow, but I doubt it'd ever disappear.