Even accounting for the cost, I think many would still argue that it was worth it, replacing a highway system that divided the city with a walkable network of parks that effectively connects different neighborhoods like financial district <-> north end, along with a new series of (underground) highways that provided improved traffic flow.
Before and after photos: https://imgur.com/JbgPur6
If the cost issue were solved, what would be the reason not to do it?
(I suppose it taking well over a decade to complete construction is an issue, but presumably new technology could improve that as well.)