Steam Hardware Survey shows us that more than 50% of Steam users game at 1080p, with less than 3% at 1440p and about half a percent at 2160p. 1440p will be 'outdated' when 2160p is the norm for gaming, and 4320p is as common as 1440p is today.
720p is becoming outdated now that 1080p holds a majority, and 1440p is becoming more popular, with 2160p being rare but still prominent. If we shift this two 'resolution generations' along, we get to the point where 1440p is becoming outdated whilst 2160p holds a clear majority, with 4320p becoming more popular and, Jesus Christ, 8640p being where 2160p is today. 16K gaming. Bloody hell.