The Cowboys aren't throwing any dice because they know that, in gambling, the odds always favor the house.
Trey Lance isn't leverage until Trey Lance can do what Dak Prescott does. Any use of Trey Lance's name in contract talks this year is pure bluff. That's not to say Trey's name won't be mentioned, Jerry will make some bold claim that Dak's agent will scoff at. Jerry won't stand his ground, he realizes the best leverage against him is his age combined with his desire to win another Super Bowl
At this particular point in his career Trey Lance couldn't beat out a 2nd year 7th round draft pick and an established 1st round bust to start for the 49ers. In order for Trey Lance to find success in the NFL, he's going to have to be developed for a few seasons, just as Tony Romo did. After two years learning behind Dak and Cooper he will be 26, the same age Romo started his first game.
Trey was brought in because he had great value that will, hopefully, prevent the team from having to endure a half dozen years of Drew Hensons and Chad Hutchinsons before finding a quality starting QB.