As elsewhere, yours is definitely not the case I was disputing but as UIs are not my area in the way you are clearly familiar with them, a question: What kind of application are you doing your UI for that can use all the cores?
Structured concurrency just means that i can both not really care where it goes, be generous on coroutines (I have only screwed up once, and that was by having hundreds of threads deadlocking on database access. Otherwise, you most likely have... 50 running at worst?). Structured concurrency also means that the moment i drop out of a screen, every computation launched in the context of this screen and not a higher level is just dropped immediately, meaning it runs just when I need it to
Actually not just juniors I also do it sometimes but shhh