This is a pretty simplistic way of looking at it. RB2 is still behind ~10 WRs and Gronk in most dynasty rankings so there is plenty of room to move up. That's not even to mention that value does not have to be tied solely to how many people they pass in the rankings. OBJ is #1 overall right now so according to what your saying his value can't go up at all, but if he goes for 2000yds and 20 TDs this year it will rise massively, even though he didn't actually move in the rankings at all.
For those baffled by the idea that Julio would be traded for Elliot right now, consider that unless Elliot has a total disaster of a year Julio will probably barely be a starting point to talks about getting Elliot next year. These guy's value peaks heading into year 2 or 3, not year 1 unless they're just really bad or suffer a major injury.
The idea that you can just wait a year to make sure he's good and then buy him for basically the same value at this time next year is, simply put, wrong according to the history of these kind of players. We have 15 years of recent history telling us that. Every one of these top 5 NFL picks at RB in that time span was more expensive to acquire in year 2 than year 1, often significantly more.