Seattle is in the midst of doing this now, tearing down the multi-level freeway that ran along the waterfront. It’s been replaced by a tunnel, and once the tear down is complete, we should have a nice waterfront area to enjoy.
Unfortunately, that double decker highway will be replaced by both the tunnel and a six lane highway in the same location as the current Viaduct.
It won't seal off the city from the waterfront as much as the Viaduct, but that's significant barrier.
Eh, the West Side Highway replacement in New York was a 6-8 lane surface road, which has been just fine; on the far side of the road from the city are very successful, busy parks and the busiest bike trail in the city.
Look at the converted elevated mass transit line in new york, the "big dig' in Boston,Duluth ,Mn i-35 extension all with parks,bike/walking trails and open spaces that people actually use. Long overdue use of pragmatic technology....
I remember biggest opposition came from store owners and street vendors who were located around the road. Having to relocate from a high traffic area would mean loss of income for the locsn vendors.