If someone is alive, then either is equally good. If someone is deceased, it would be less common to say "I have seen" than "I saw", but it would not be creepy or inappropriate - just less common. In most cases, whether you gravitate toward simple past or more complex past tenses depends on your own personal ways of speaking more than a "right/wrong" issue :). I hope that helps! Great question!

You could say "I have seen Michael Jackson live" even though simple past is more common. You're perhaps in this case talking about an experience you have had amongst other experiences, for example "I've seen Elvis, Beyonce, Elton John...". But simple past works better here with questions: "Did you (ever) see Elvis live?" fits more with the idea that the window of opportunity for this is in the past.

Whether M.J. is alive or dead has nothing to do with the choice of verb tense. You can say "I have seen" or "I saw" because YOU are the subject of the sentence and YOU are alive. However, if M.J. were the subject of the sentence, then it would matter. Since M.J. is dead, in most cases you would not say "M.J. has performed in New York" You would instead say "M.J. performed in New York". The logic is simple. Since M.J. is dead, he cannot "have" anything. He cannot have a toothbrush, and neither can he have performed.

In most cases these two versions are interchangeable, but it can also slightly emphasize different details: "I've seen Michael Jackson" - you point out your experience, " I saw Michael Jackson" - you just state the fact that happened in your past, without highlighting it as your actual and up-to-date experience by now

You're right that when referring to someone who has passed away, we typically use the simple past tense. This usage is because we often talk about deceased people’s actions or experiences as part of a closed, completed past. So, for someone famous who has passed away: "I saw Michael Jackson in concert." (simple past) "Did you see an Elvis live performance?" Using "I've seen Michael Jackson in concert" or "Have you seen an Elvis live performance?" sounds less natural because present perfect tense often implies an ongoing connection to the present (like “He’s still alive, so it’s still relevant now”). For deceased people, simple past tense keeps the focus on completed actions and tends to sound most natural in English.