Elliott is in a great situation and he knows it. Garrett's offense can no doubt be frustrating, but Zeke is running behind a top 3 OL and there probably aren't 5 other teams willing to feed an RB until he pukes.
KC won't trade for him. Andy Reid's been a head coach for 20 years and I think the highest pick he's ever spent on an RB is a 2nd rounder for Shady McCoy. His offense needs backs who can catch short screen passes more than anything, so he does fine investing in speed-deficient RBs with good hands like Westbrook and Kareem Hunt. He doesn't want an all-world bell cow.
So despite the posturing, I think both sides want Zeke to be here. Garrett's conservative (to put it nicely) offense has functioned at its best when he has a power back who can control TOP and pick up short yardage, and Zeke leaving a maximum volume, great blocking situation at age 24 would be a bit like Eric Dickerson going to the Colts. Maybe if he is all about the money, he would take a big contract from one of the Oaklands or Tampa Bays of the league, but I think what Zeke really loves is being the star. He'd rather be in the limelight winning rushing titles for America's team than grinding out 1000 yard seasons in relative obscurity in Jacksonville.
Now he's not going to accept a DeMarco Murray type screw-you offer to do it (4 years $16M for the reigning OPOY... really), but for a market value $14M-$16M yearly contract, I think there is plenty of motivation for both sides to sign that deal. What they are likely haggling over now is how much is guaranteed and when that money will be paid. Zeke wants it now, Dallas wants to pay him after Dak and Coop are taken care of.
It's just like the D-Law contract drama we just went through. Lawrence claimed he wanted to be paid more than Mack and hardballed the team by refusing to do surgery on his shoulder, but when push came to shove, he was calling Stephen Jones directly on the phone and said "I want to be here, let's make this work." Zeke is hanging out in Mexico now not talking to the team, Jerry is signing washups and saying "we don't need you to win a SB", but closer to the season, something might get done. Either they will sign him to a big money extension, or give him a raise for 2019 and talk again after the season.