Here are a few channels that I watch ofen that always upload in 4k. This is not a niche for serious youtube channels:
https://www.youtube.com/channel/UCsn6cjffsvyOZCZxvGoJxGg
https://www.youtube.com/channel/UCHnyfMqiRRG1u-2MsSQLbXA
https://www.youtube.com/channel/UCSpFnDQr88xCZ80N-X7t0nQ
https://www.youtube.com/channel/UCXuqSBlHAE6Xw-yeJA0Tunw
Besides even at 1080p the difference in how much battery it saves is astounding. With acceleration your CPU is just idling while without it has to work quite hard.
If they have a setup that can display and content that they want to watch in 4K without their laptop burning a mark on their lap, then that's a valid use case. An use case that doesn't work with Firefox at the moment because of Firefox. "You're holding it wrong" is an incredibly inane attitude.
I use mpv when I am serious, moving FF entirely out of the picture.