if it saves the taxpayer money now and in the future,. that would be a good thing.
That’s the point. It doesn’t. It just ensures the public expenditure is ugly.
If it’s Design-Build, the contractor will pocket the differential. If it’s Design-Bid-Build, you end up with cut and pasted designs without any design wherewithal to adress actual design.
It’s like making a regulation that all maladies are prescribed aspirin to save medical treatment costs.