There are a lot of people unlike you, and please don't assume that they all have the same patience that you can give to Youtube.
Well, iPhone XS Max is 6.5 inch, Galaxy Note 9 is 6.3 inch, Galaxy S10 Plus is 6.4 inch. Most phones used in South Korea are at least 5 inch or bigger.
Korea's 1440x2960 counts 11.68% out of all smartphone users, 1440x2560 counts 19.03%. 1080x1920 counts 29.21%.
That's 60% of all smartphone users.
Then, there comes the need for 1080p/HDR mobile videos (think Netflix!).
Awesome. But the article is about the United States.
Korea's 1440x2960 counts 11.68% out of all smartphone users
2960x1440 on a 6.5-inch phone is 506dpi, which is more than the human eye can see. So you're either a marketing victim, or trying to impress your friends, or both.
I stand by my assertion that you don't "need" (your word) 4K on a phone.
So, no, unless you are an insect, you cannot tell the difference between 720p and 4K on a mobile phone. It's all just showing off for your friends.
Here's something that people never say anymore: So what?
Who cares if YouTube lags on 4G? So what if you have to wait eight seconds for buffering. Does it really matter if you see SD or HD video instead of 4K video on a three-inch cell phone screen?
No, it doesn't.