Think different.
Look at the 12" MacBook which stands alone as a "future of notebook design" going on its 4th year now virtually unchanged and with features that haven't moved down the line to the other MacBook's. The Smartphone market is very crowded and Apple stands alone against 25+ Android competitors and in an age where phones are no longer $0 on a contract.
So having a top-of-the-line iPhone X Series with demonstrable feature differences from the plebeian 4/5/6/7/8 Series makes a ton of sense, no different than how Mercedes has it's C/E/S Class or BMW has its 3/5/7 Classes. That's what luxury brands do. They make one basic item (a car) and then differentiate them (size, features, price).