I’m currently running it on a iPhone 6. If I upgraded to the iphone 8 or even iPhone X would the quality go up automatically? Also I’m displaying it using airplay on an older model Apple TV.
It will slightly improve, but its not going to make you have a “wow” factor like using hardware with a dedicated video card.
Just got my Iphone X after having the 6+ and i can confirm the picture does improve greatly with more detail , colours and depth. As my laptop cant run zwift i just hook up an HDMI converter onto the iphone and plug straight into my 4k samsung tv and its wow. Remembering to change the picture quality in the zwift settings once you start a ride ,it does normally remember the setting but i always do a quick check first.