Economy or premium economy on the upper deck of a 747-400 really isn't that unheard of.
Both Virgin Atlantic and Air New Zealand split the upper deck between J and Y. They don't seem to have an issue with serving business class out of the galley at the back of the upper deck, passing the J carts through Y.
AF also has Y upstairs. Not a bad ride.
It's hard to fault Delta for trying to match cabin capacity with real demand, not filling seats with upgrades or non-revs. DL's revenue management is quite good — if you pay business class fares, I'm sure you'll still find a last-minute seat.