In the first case, rudeness can be helpful and in the second case it won't hurt their feelings. Seems like a fine choice to me.
Assuming you meant 'But you have to be brutal to have the desired effect of getting through to the terrible singers, plain honesty - considerate, respectful honesty[0] wouldn't work', well, he didn't get through, did he. Instead they thought he must be just being rude deliberately and didn't mean what he said. He made it impossible for them to hear him. Mostly.
Anyway.. I was making another point, not that Simon became notable 'simply because he was honest' but because he was rude; whether it was effective (at getting terrible singers to realize they're terrible) is another thing.
[0] See the book Crucial Conversations, it's largely about how to be honest with people, in important situations people usually aren't, while being respectful and considerate.
Are you implying those people are auditioning ironically and know they are bad singers?
It does not seem rude to me to try and change the course of someone's life who is _drastically_ underperforming.
Is the definition of rude hurting someone's feelings? That doesn't follow the implication that rudeness shouldn't happen; sometimes people need their feelings hurt to inspire positive change.
It does not seem rude to me to try and change the course of someone's life who is _drastically_ underperforming. - Leaving aside what underperforming could mean - so, if he means well, it's just not possible for Cowell to be rude, no matter what he says? I think that's just not what the word means to people (apart from you).
That doesn't follow the implication that rudeness shouldn't happen; - I can't parse that, sorry.
In reality, people often have to be forcibly dethroned from their views. Having a judge say no thanks will not have the appropriate effect.
The issue is emotional commitment, not accurate assessment.