You'd said the same thing about Mahomes backing up Alex Smith.
Not hating on Lance but this is not the same thing. Mahomes played a LOT more college ball and not only that, he did so at a much higher level of competition than Lance.
Everything about Lance is projection based on the fact he's a strong armed kid with great athleticism. Does he have the talent and ability to be a franchise quarterback? Yes. Then again so did Josh Rosen and Zack Wilson.
Having said that, if I am the Cowboys I would not let Lance go until it's determined what he is. And I am giving him every single snap this preseason because they need an answer.