Even if Hayward is put in as the SG of the future for the Jazz, why do we assume that everyone we draft is going to start and finish their career with the Jazz?
We need shooters, desperately. If Thompson is a good shooter, and arguably the BPA at 12, lets take him! He can come off the bench. Having too many shooters is a good thing.
If Thompson comes in and is a really good player, our bench is strengthened. Perhaps he overcomes Hayward as the #1 SG for our team. No one knows if Thompson might actually turn out to be better than Hayward. Or perhaps Hayward has a career far better than Thompson. We don't know yet!
One of them could be traded. Or we can keep both of them and create mismatches.
The point is, we shouldn't draft inferior players because we "already have" Hayward. I'm getting tired of people suggesting this.
Go for the best player available, strengthen our team, let everyone compete for their minutes, and later trade them for other parts (if necessary).