What happens to retailers?
Perhaps, the retailers will sell them at a discounted price, for those players that only want the core game. Some people prefer to spend a little to try out an older part of a franchise, before purchasing the more current offerings. /shrug
They’re not completely worthless. They are still available as the game is still running. The game isn’t going F2P like Wildstar, so they’re not going to pull those versions. There are still those out there that would like to rather pay $20-30 to try the game’s core rather than spending $50 for the whole package of HoT. And no, they won’t automatically upgrade to HoT (that makes no logical sense at all because of the price difference).
Send retailers a bunch of stickers saying “Now includes expansion”.