Honestly, Henry and Zeke aren't that valuable either. The real question will be McCaffrey, because he's an elite RB, and a top receiver.
I kinda think the bolded has already happened. We've seen several WR's that would have been RB's 20+ years ago. I certainly think Deebo Samuel would have been, probably Laviska Shenault too. Conversely, I wonder if he were coming out now instead of 1994, if Marshall Faulk would have been a WR?
One issue to this idea fully taking hold, is that the lower you get on the level of play, the more valuable RB becomes. Its probably your best or 2nd best player on HS teams.
I know if I had a really athletic kid who wanted to play football, and wasn't going to be a QB, I'd be trying to get them to play CB. The money is there, and the demand seems to be far greater than the supply. Which is how guy like Trey Waynes get 12 million a year, despite being a pretty below average starter.
That is a really bad idea. Most 18 year olds, even the most athletically gifted, are still going to be physically unable to handle playing with NFL players. All skipping college does is leave them with no education. Coming into the league sooner doesn't change the value of the position, if anything it would make it even lower, as the supply of RB's would get even bigger.
The best answer is that the truly elite athletes should play other positions. RB should be to WR, what G is to T, where its often the place guys get moved when they can't cut it at the more important spot.