RFK is on federal land. So the federal government has to approve any use. And each time the federal government changes, the new administration can impose whatever demands it wants on the team before it is allowed to build a new stadium there.
Likewise, you will have local politicians who want to build a beautiful new stadium there, local politicians who would prefer to see housing or a new commercial development there, and local residents and voters who have opinions of their own.
With all the people who would have to agree on a new use for this massive site, I am pessimistic that anything will get done.