Depends on your definition of significant and sidewalk. It does add cost, so it's not insignificant (you can observe it with confidence). For states that are already struggling with infrastructure costs, it would not be feasible (look at PA and it's bridges).
Most states or municipalities actually have codes that the sidewalk has to follow. So it it's not legally possible to build something low quality. Technically a sidewalk doesn't have to be paved at all. In many places, you have a 3-6 foot right of way on the sides of the roads - aka a sidewalk - that does not have to be paved. This can vary by state and municipality though.
What a bad-faith definition you're using here. By this definition if you reimbursed a road worker for a penny they dropped during their shift building the road it'd be a significant expense.
Do you have any actual data to support the idea that sidewalks for all roads are a feasible goal?
That is complete nonsense. Sidewalks are not a significant additional expense to the road itβs on, even at a much lower level of build quality / complexity require very little maintenance as the only damage is environmental (pedestrians and cyclists do 6 or 7 orders of magnitude less damage to the surface than sedans do, per unit of distance, add 4 for a loaded truck).