I don't think 4k120 today is in the same market position as 1080p60 5 years ago. 5 years ago 1080p60 was the bar, if your device couldn't do 1080p60, it wasn't a gaming product, it was a business product. I don't think 4k120 is there, I think we're just at the bar where 1440p60 is considered at that level. In that time, the higher end target has moved from 1440p60 or 4k30 to 1440p120 or 4k60, which means the halo products now have to promise 4k120 to keep ahead of the merely high end.
I think the current increase in interest in high refresh rate gaming came from the shift away from TN panels. Once IPS became reasonably available, companies with TN products couldn't push for them as premium products from an image quality perspective, but high refresh rate was something they could still market with TN that there was a window where IPS and VA could not do that. So they did, gamers got exposed to high refresh rates and then moved those expectations to other products.
Also tech like gsync/freesync meant that if your monitor had a higher refresh rate than your hardware could produce, you no longer had to deal with tearing.
I think monitor tech is leading GPU tech at the moment, with 1440p240 and 4k120 monitors. There's certainly _some_ games where those resolution/framerate combinations are achievable, mostly esports titles and older/smaller titles, but for the most part, no (also for me at least, past like 90hz you're into dimishing returns territory). But now people have these monitors capable of X resolution and Y refresh rate, people want to have their cake and eat it, wanting games that have modern effects for higher fidelity and yet also higher fps to take advantage of their monitors.