The initial improvement wouldnt be that the QB is better than Dak it'll be that we have a lot of CAP space in a year when it's expected a fair few players will be cut as teams try and get under the restricted cut.
Lets be honest Dak is good, but he's not Mahomes, Wilson etc and he's got one of the best offenses in the game.
That CAP space isn't going to help out the team much if Lawerence is a bust or doesn't end up as he is hyped up to be, which has happened before with top QB top picks. The Cowboys front office still better be sure that Lawerence is better than or at least good enough to make the Cowboys consistent playoff contenders. It's a risky move especially if the Cowboys plan to trade up to get him.
Either it could be the right move and Lawerence works out to make the Cowboys playoffs consistent contenders or he won't and the Cowboys will be right back to having to eventually look for another QB or hope that they can land a QB who has been playing as good as Dak has been. Knowing Cowboys fans, they'll be upset at the front office, even though the front office did what many Cowboys fans wanted them to do.
And if Lawerence is what he is hyped up to be then the Cowboys would have to pay him eventually, maybe even more money than Dak with how contracts keep going up. Does the same logic of paying Dak apply to paying Lawerence in that a team can't keep good players around if they have to pay the QB a big contract? What happens if the Cowboys don't win a Superbowl while Lawerence is still on a cheap contract?
Still better be sure that tanking for Lawerence is the correct move for the Cowboys future.