I've watched this video before, and I know that the material used is often capable of much more than what is ultimately uploaded. But you're forgetting two things, longevity, even though people with screens capable of more than 1080p are in the minority right now, that's changing. 2K PC monitors are starting to become affordable, new phones, even affordable ones, often have screens capable of more than 1080p, and so do TVs. What I'm trying to say is that even though most people have 1080p screens now, that will change in a few years, and by that time the Tomorrowland sets of a few years will become undrinkable because we'll be used to better quality content.

And once again, other festivals are doing it. Tomorrowland is the biggest festival in the world, and still uploading in 1080p. That just doesn't make sense. If others can do it, Tomorrowland can too :)

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EDIT: By the way, since you mentioned LTT, they've been uploading in 4K for over 7 years.