What could be done instead:
Develop RFK area

Air rights platforms over swaths of the existing parking lot areas would preserve parking capacity while providing new space for new developement, either with little or no excavation.

Air rights platforms over the below ground SE Freeway/Barney Circle Underpass extension in the area around Massachusetts Avenue

Up front purchases of some areas of existing residences which would be replaced with new development that would include replacement dwellings for any such displaced residents who prefer to remain in the same local area. This would not use eminent domain, as it falls outside the traditional understanding of the U.S. Constitution's scope for its use for public purposes, such as roads, parks and government buildings, but not for more private uses as redevelopment. Instead, it would use financial inducements, perhaps 3 X market value.

If used for an extension of the existing East Capitol Street Plaza, this would show a popular appreciation for acquiescing to a public right of way.