If Apple is really out of the standalone display business, here is the logic I can think of:
1.) Given the dearth of any 5K monitors, and the fact that it also comes with a powerful computer inside, the 5K iMac is an incredible deal for those wanting a big Retina desktop experience. Dell's standalone 5K display costs ~$1,500 with the 5K iMac...a quad-core computer...starting at $1,799.
2.) The advancement of iCloud Drive, specifically with macOS Sierra also syncing Documents and Desktop folders, makes owning and using multiple Macs very close to seamless, and in many cases, better than having a laptop+monitor setup.
3.) The Thunderbolt Display is really the only product Apple made without a CPU in it. iMac, MacBooks, iPads, iPhones, and even iPods are all computers in some way. Aside from small peripherals like headphones, keyboards, and a mouse/trackpad, Apple only makes computers.