The questions described don't sound so much like existential questions as rhetorical ones. They are not asked to get an answer but to make a point, that point b

The questions described don't sound so much like existential questions as rhetorical ones. They are not asked to get an answer but to make a point, that point being that the questioner believes their friend's startup idea is a poor one. So yes, it's generally better to engage with someone you want to persuade by asking actual questions, rather than talking at them with rhetorical ones.