Trey Lance is a more athletic version of Dak at the beginning of his career. Same basic problems -- bad mechanics and footwork results in too many passes being high, low, and behind. Just like they did with Dak, defenses would clog the middle of the field and force Trey to throw to the outside and he would struggle.
Dak has been able to correct these issues over many seasons and become a solid QB. Why do I want to watch us go through that growing process again? Dak gets a season ending injury then sure I would roll the dice with Lance over Rush, and maybe that turns into the same kind of situation Dak found himself in when Romo got injured. But barring that, I'd rather Lance work through these issues as the 3rd QB. Maybe he turns into Dak's eventual replacement, get a compensation pick, or we can trade him for more than we traded him for.
But if we are talking about trying to win a Super Bowl any time soon Dak, even with his limitations (including cost), is our best shot.