Why is that true. Why does he have to be HOF caliber hitter to justify the 3 overall pick? Do you have the same threshold for a pitcher? A corner OF? A team needs a 1B, and I get that guys can be moved into 1B if they can't play elsewhere, but if you are drafting a guy expecting that he will have to be moved, how is that different?
Let's say you draft a big mashing 3B, that most people believe will have to move to 1B, sure there is a chance he can play 3B, but it seems most likely you just drafted a 1B. Is that much different?
It is easy to say just draft up the middle talent. But for the most part, in the history of baseball (CF excluded, actually) those guys aren't the best hitters. Your organization needs guys all over. You need SS, but you also need 3 hitter. If those can be the same guy, well that is a huge bonus, but that hasn't happened much in the history of baseball.
Witt (and I am no expert here, just for the record) is an up the middle defensive player correct? But he has some concerns about his hit tool correct? So his defensive abilities may be completely negated by his lack of being able to hit. So you get maybe a utility IF out of a 3 overall pick. That doesn't really do much for you.
Everyone here is high on Rutschman, and he seems like a monster, but playing catcher wears guys out. It drains them of offensive ability, at least usually. Maybe he is the 1% of guys like Molina or Piazza, but even guys that looked like studs in Mauer and Posey get worn out and get moved to 1B.
From 2000-2010 (from a fangraphs article (https://community.fangraphs.com/success-rate-of-mlb-first-round-draft-picks-by-slot/)), guys taken 1-5 have an average WAR of 12.8 (article written in 2016, so some likely still accruing), does a 1B need to get higher WAR than that to be successful? No. Do they need to hit more? Yeah, a bit. But that player wouldn't have to be HOF level to justify the pick. If you got a Jose Abreu out of the 3 pick, well, I know I'd be thrilled. Eric Hosmer was the 3rd overall pick, I would bet the Royals were happy with him, even though he wasn't as good as Frank Thomas.