Our history is replete with compromises, give and take away. From the Founding Fathers to the president, congress, and courts of today this process continues - a whittle & nick here, a reversal there.
Example the libs/antis were given the assault weapon ban, then had it taken away.
Conservatives/pro gun people were allowed to buy new full auto guns, then had that taken away.
All three branches of our government have hacked at the second Amendment, taking away/reducing, restricting our RKBA. You are right that a future congress could do more - better or even worse laws could be passed.
Some problems are perhaps too overwhelming and/or involved to be solved by mutual consent of the states. I don't think this one qualifies, falls in that category.
"What the government gives, the government can take away" is not just a nice turn of words. It is the caged beast, clawing to get out.
We celebrate Heller and McDonald as far as they go, but are reticent to have congress act within those rulings? Makes no sense to me. Would we have been unhappy had the decisions gone farther and better protected our RKBA to posses usable guns in public?
For sake of argument, let's say it was a straight up, "shall not be infringed" no permit needed, just a straightforward declaration that "no citizen of the United States age 18 or older shall be prevented from carrying a firearm in public for his self-defense...." with appropriate redress to federal courts against any local or State official who attempted to enforce any law to the contrary.
What makes that any more of a concern at the federal level, than it would be at any State level?