I don't think he's advocating that we stick to 5k but rather that 5k is good enough for today's devices and as they improve we can up the resolution.

Advocating for higher than necessary resolutions today based on future prospects is a bad gamble because consumer adoption is still in it's infancy. What if you make a poor user experience today in anticipation of a better experience tomorrow but instead kill the market so there is no tomorrow?

Video game developers and video content providers are both well versed in dynamically scaling to match the capabilities of the consumer.