Yeah, that is the question. Some people will be happy to pay for the best consoles and higher resolutions, but if it still plays fine on the old model most people won't switch.
A lot of the reason behind Xbox One X is to increase mind share, have the best high-end console and hopefully gain low-end Xbox One S sales on the back of that.
One other key feature of the Xbox One X is faster load times in older Xbox One games (or any game that lacks X support) as it will use its extra RAM as a storage cache for faster load times.
Long story short is that only certain people will be attracted to the X. Though it will also make the standard model look very cheap by comparison, which helps.