DNAinfo was never profitable in its 8 year existence. With the union taking root it was probably never going to see a return.
In my opinion the only unethical thing here was taking down the archives. Total severance seemed fine.
What about Gothamist? I would bet it was profitable since it was run independently for a long time. It was a glorified blog — lots of reprinting Post articles rather than the independent reporting that DNA Info did, so I imagine it had much lower costs.
This tweet by someone who appears to have written for Gothamist claims that "Gothamist actually WAS profitable. DNAInfo wasn't."
They could have sold to a company that would have overhauled their monetization, but you're right that it isn't _unethical_ to shut-down (unnecessary, maybe).
Opex loses is liability. Company with no opex is an asset.