Where I'm going with this is that there is no reason to be all that concerned about the RB position (I thought this was pretty clear), because #1 - in the modern NFL the position really does not matter that much, and #2 - what they've already got is a good enough group of players.
None of the options remaining on the market are really any better than what Dallas already has on the roster, and some washed vet like Leonard Fournette is not going to move the needle.
Also, in 2015 didn't Darren McFadden rush for something like the 6th most yards in the entire league? If anything I think that kind of goes to show that good-enough RB play can pretty much be found anywhere, even in a lost season like 2015 when defenses were loading up the box to tee-off on an offense led by Mat Cassel, Brandon Weeden, Kellen Moore, and co.