Apple does 120Hz different than Android, they have ProMotion, which uses a variable refresh rate. It can lower the refresh rate to 24Hz when the screen is mostly static, actually giving you better battery life, then increase it up to 120Hz when needed. Look at how the iPad does 120Hz, it didn't loose any battery life and didn't need a bigger battery.I had an Android phone with a 120Hz display and even with a 4000mAh battery setting the display to 120Hz would drain the thing in a few hours, so I completely understand why Apple is not going to ship this feature. Anybody who says they want this feature does not understand its impact on battery life and would not want to use this feature if they had it on an iPhone.
This is probably the reason the feature might be delayed. Apple wants to get the auto refresh rate thing down properly so that the user doesn't have to turn on or off an option in settings.