Some time ago I disabled 5GHz wifi (1300Mbps). I simply did not need it. All my smart home appliances run on 2,4GHz. Gaming PCs are on cable for minimum latency. And cellphones/laptops are in no need or able to saturate 600Mbps I have available on 2,4GHz in between themselves and NAS like devices. As far as connection to outside goes, its 150/15Mbps will be saturated long before wireless. Not saying that some people are not able to use such bandwidth internally. But for most homes, even if all devices did strain network in worst way available in given home at same time, Wireless would not be limiting factor. And If I had like servers which would talk this amount of data between each other, I would not use shared medium like air. Cable is cheap and reliable. And Air would remain free for heavy utilization.

The reason to use both bands has little to do with bandwidth, it has to do with collision domains. Despite all of the advances made in the last decade with MU-MIMO, beamforming, mesh networks and the like, each wireless network still uses the exact same TCP/IP stack that it did when WiFi launched. That means that your entire wireless network remains a single half-duplex collision domain, able to process only a single packet at a time. When multiple devices are sending data at the same time, the packets are handled on a first come, first served basis, and the rest are dropped and need to be re-sent by the devices. Hence, the more devices you have, the more latency every device on the network has due to dropped packets needing to be re-sent. Your average smart home device is chatty as hell, sending keep-alive packets pretty much constantly in order to be able to respond instantly when called. I've seen people with 20+ WiFi smart bulbs in their homes complaining that their router sucks and they can't stream video smoothly even standing right next to it, and people who add Google WiFi with no dedicated backhaul and suddenly their network is even worse. Understanding this one basic tenet of how wireless networks work is probably more important than any other. It only comes into play when you have a lot of devices and a mid-range router with a slower processor, but that description is rapidly becoming the norm. So why should you turn on the 5Ghz band when the 2.4 is plenty fast? To keep the shit you actually want to use away from your chatty appliances who make it difficult for you to keep a steady UDP stream going.