Ultimately, the decision will be up to the people of NYC
and I'll support whatever decision they make.
My opinion, for what little it's worth, is that a memorial is appropriate but shouldn't be the sole thing on the site. After all, we didn't close Pearl Harbor and make the entire place a memorial. We did build the Arizona Memorial (and the much lessor known Utah Memorial) to honor those killed in the attach but the harbor remains open.
On the other hand, several major battlefields from the Civil War were set aside and never developed. However, those areas hadn't been developed before the battle, either.