This is me. I'm not a "wait and see" RB guy.
It's McKinnon's 3 down show in SF. Chubb's got Hyde & Duke ahead of him. Duke is significant pass game obstacle. Hyde IMO is at least a 2018 obstacle. I wouldn't be surprised if the pick could just be turned into Chubb+ after the season.
		
		
	 
	
		
	
	
		
		
			I'd need a first the other way to give turn McKinnon into Chubb.
		
		
	 
Here's the thing, though.  McKinnon is already 26.  Like it or not, he's almost assuredly a depreciating asset regardless of what he does after this year.
Best case scenario, McKinnon puts up top 5-10 RB numbers (let's face it, he's not outscoring Gurley, Bell, D. Johnson, Zeke, Fournette, Barkley and I can probably list a couple others as well).  And even if he does that, his value almost definitely won't go up from where it is now when he's entering his age 27 year.  At best, it's the same where it is now.
Looking at Chubb, worst case scenario is Hyde and Duke get most of the playing time this year and Chubb doesn't get a chance to do anything.  But, heading into 2019, Duke is a free agent next year and Hyde will be entering his age 29 season.  Chubb still retains a ton of value without doing anything.  Or even if he does and doesn't do much, he's a good enough prospect that he gets a pass after just one year (look at Mixon).
Given those 2 scenarios (best case for JM and worst case for Chubb), it's very likely Chubb has the same or very similar value as McKinnon just one year from now. 
However, in almost any other scenario, Chubb > McKinnon in as little as one year, if not earlier.  If McKinnon is just average (or even somewhat above average but not great), gets hurt, loses his job, or SF drafts someone next year (because it's not as if JM is a non-replaceable stud talent) AND/OR Chubb does anything this year and then it's a massive loss for the McKinnon side. 
This is a dynasty thread and 1 year in dynasty is a relatively short time.  This isn't a matter of having to wait 2-3 years for situations to likely change.  So unless McKinnon is the difference between winning a title than not this year (if you were to have kept Chubb), then paying Chubb+ for J. McKinnon is going to lose 9/10 times.  Look at recent highly drafted RBs that had to sit behind vets or didn't do much (D. Henry in 2016, Mixon in 2017), and their value has only gone up as time has passed.  Yet recent similar 26 yo RBs with starting jobs that actually did well (Hyde, L. Murray) have only gone down, especially when their teams drafted guys to replace them.
IMO, given the value I've seen him fetch, McKinnon is probably the most ideal sell-high of any player heading into 2018 before he steps foot on the field.  It's amazing what he's getting as a guy who's averaged 3.8 and 3.4 ypc the last 2 years, career 4.0 ypc, has NEVER had 160 carries in a single season and has never scored more than 3 TDs now heading into his 5th year.  Who is 26.  Who wasn't drafted that high.  Who is going to a team that was in last place in the NFC West, a very difficult defensive division.  He is just SCREAMING to be sold this offseason for the prices he's fetching.