Dave is a very smart guy and knows quite a lot about baseball, so I find myself rather stunned by his position on this.
Reb is right. Dave is wrong.
That said, yes, as an example, Kris Bryant could probably play OF for five years and move back to 3B afterward to play what would likely be very bad defense by major league standards. Would two out of every ten chances result in an error? Probably not - as Curt said, he's a major league baseball player and a fine athlete. But there's a reason we almost never see this and, when we do, it's driven by extraordinary circumstances.
If Dave's position is limited to what I describe above, fine. If, for example, he thinks that Addison Russell could play first base for five years and then move back to shortstop and once again become a top-flight defender, sorry but there's not much evidence to defend such a contention.