That proposal is as stupid as it could possible be, it is so dumb that makes me wanna cry. As if the problem with the NYC Subway were the trains. Will the buses still run in flooded tunnels? Who will pay for retrofiting hundred of kilometers of tunnels and elevated viaducts for being able for buses to circulate? Who will pay for throwing away hundreds of subway cars and replacing them with thousands of buses? Even supposing that the same capacity could be achieved using buses instead of trains, you will spend billions of dollars in that to basically left the system just as it is. Yes, brilliant.
The problem with the NY subway are not the trains, that can be replaced regularly, it is the century-old infraestructure, which is not that easily replaceable. The same aging infraestructure use by the trains, would be used by those hypotetical buses, still having the same problems.