Every quarterback in the NFL is drafted with the hope that, eventually, he will replace the starter. At the very least, the competition will push the starter. This is what competitive sports is all about.
Roger Staubach actually admitted, once, that he pushed himself to improve due the high regard he had for Danny White. Statistically, Roger was the #1 ranked QB in NFL history and had led the Cowboys to four Super Bowls at the time he said that. When Danny White opted for the WFL after the 1974 draft, Roger's backups were Craig Morton, a former #1 pick and Clint Longley, who cost the Chargers their #1 overall pick when they took him in the supplemental draft.
If any starting QB is afraid to compete then what does that say about confidence and ability to lead your team? Dak should welcome the competition and an opportunity to confirm, that he is the rightful starter.
However, this is all a mute point. Trey Lance isn't even ready to compete for the backup job. He has thrown less than 400 passes when you combine both his college and pro careers. Fans should also consider whether Jerry Jones feels he has enough time left to make a change of that magnitude and still have a true Super Bowl contender.
Today, Trey Lance has the potential to be next preseason's superstar. that is when we will all get to see him on the field and that's the way it should be. He is not going to replace Dak this season. He is not going to replace Cooper this season.