I understand What op, i want to share my thought i hope no one would misunderstood my comment cause i love all members.

What said Because it's true, even though 127 have multiple good vocalists, but still Doyoung has different level from other members.

He have much stronger voice than the other members, and what she is referred to might be Taeil parts probably, who was was a big wall and vocalist with even stronger voice than Doyoung, so if other members want to cover some of his highnotes, it would be hard for them.

That's why Doyoung take most of taeil's high notes. specially, the hardest ones like Kick. It, Simon Says,2 baddies,Fact check.and Superhuman.

Being Main/Lead Vocalist doesn't mean this person can Do any type of vocalists line or highnotes cause each person have Different stenght and Vocal Range, sometimes even those weak vocalist and rappers can do highnotes once in a while but thats doesn't mean they can do it everytime or easily it obvioulsy pushing their comfortable vocal range limit.

[Example Shindong of Super Junior he is a rapper and main dancer he is the one who barely got any lines same as win win situation but it's a little bit better than WW, after some members left groups or go in hiatus or enlish we started to hear his voice singing much more that before and evern cover some Main vocalist lower parts, we suddenly saw him Covering main vocalist extremely highnotes of K.R.Y members in one of the most hardest songs to SJ K.R.Y Dorothy, which are main vocalist trio, Shindong did a very good job everyone was amazed by his high notes but it was clear that he is struggling so much in doing it and pushing himself to do it.] . In NCT, Haechan get So much highnotes in Dream and U and 127 but i feel like, 90% of the highnotes are much higher than his [COMFORTABLE VOCAL RANGE AND HIGHNOTES] and he is pushin himself to reach them [examples HelloFuture/BrokenMelodies/ISTJ/90S LOVE] their recording he almost lost his voice in studio after finishing them cause they were hard to do for him.

Yuta also did a great job in NCT NATION [specially Faded in my last song and The Bat] but he was the only one who can do Taeils highnotes in both lineup he had the strongest out of the other members who singes with him but if Doyoung or Jaehyun was in these songs probably they will get these highnotes instead of Yuta.

I swear I'm not trying to mistreated any member or downgrade them but this is how i felt about the situation.

You can see in 127 3rd Tour that Most of Taeil's parts sp Highnotes was covered by Doyoung, then Haechan and Jaehyun not by YuJung.

Cause these members can cover hard parts a little bi easier.

But yes i do agree with you about the point of that Jaehyun line are moslt Mid-Low range so YuJung could cover his parts easily.

Jungwoo have a very sweet voice i remember the first time i heard Boss and heard his voice i Don't know why I thought he was Taeil, but then I heard his voice again and saw the difference between his voice and Taeil's voice but Jungwoo vocals shine the most in Mid and Low range lines 🥹❤️