I'm slightly surprised there already hasn't been a PlayStation or Xbox without an optical drive at this point honestly. I imagined there would be a digital-only PS3 or Xbox 360 late in their lifespans. A ton of the PS3's retail library and certainly a large chunk of the 360's retail library is available digitally, plus huge libraries of digital-only games. Sure you'd miss out on some retail games, but each system has big enough digital library to warrant a cheap console model.
Now that we're in a state where every game get's at least a digital release, I always imagined at least one of the two companies would push out a model without an optical drive this generation, especially if they can significantly drop costs. What if dropping it allowed Microsoft to beat Sony to the $299 price point?
Another reason to do this would be as an excuse for Microsoft to push digital harder than before. If it knows it has customers who are absolutely digital-only and engaged with the Xbox store, it'll try to push more products in the eyes to those players. One reason Valve is so much more aggressive with its digital distribution (frequent sales, pre-order bonuses, four-packs, free weekends, special events, etc.) is because it doesn't have to worry about retail at all. At least Microsoft would have some segment of its user base for which it doesn't have to worry about retail.
And at this stage a digital-only console should have a sticker on the box that says "Minimum 100Mbp/s Internet Connection Strongly Reccomended." Too many people still have shitty internet and data caps for someone to launch an entire console that's digital-only, but a digital-only SKU would at least be convenient for people who have fast internet. If my math isn't off 100Mbp/s should be enough to download a 40 or 50GB game in under half an hour.