Some environmental impacts can be fixed later, but it generally much more expensive to "bolt-on" a fix than to design the project properly in the first place. This of course means the fix is rarely implemented.

According to NEPA, when an impact is identified, the intent is to develop an alternative to avoid the impact entirely. It's only when alternatives are found to be infeasible that other mitigation methods can be considered.

The SF bay area is still fixing environmental damage from 50 years ago (semiconductors) or even 170 years ago (gold rush/mercury mines). Environmental problems can outlive everyone that will decide this issue.

These are permanent solutions to a temporary problem. No one is digging up the roads and pipelines after the economy has recovered.

I think you've gotten confused - my question was whether they are indeed permanent solutions, or if we can fix them later without having to dig the whole thing up.