720p is still used quite a bit.
A person might buy a 4k TV, but they likely will still own a number of other displays and devices. So really, in 2017, 4K/UHD is a gimmick for MOST releases. It's likely we'll see games and movies in 2 years that look better in 1080p than today's 4K games because the tech hurdles will be chipped away and optimizations will be present. More is better, but my point is even lower resolutions benefit from technological progression.
4K is still an early adopter tech and the displays are still considerably less reliable and are somewhat power hungry. Says the guy who owns a number of UHD devices since 2014.
High refresh and draw distances are really the superior experience in gaming over resolution. Once large form factor UHD 75Hz+ displays that match today's power usage from an HD TV of similar size wind up making up 30% of new TV/monitor sales, then I would say it's basically time to take a harder stance on phasing out old displays in your home.
That being said, you can upgrade as soon as you can comfortably afford whatever, but I'm just making a point about matching a cost:benefit curve.