I'm a bit confused what benefits buses have at that point - you would need to dig a dedicated subway tunnel (and maybe spend additional to widen the tunnel so that buses can navigate it more easily) so you're out all the construction costs - you're just using a vehicle that has the ability (which you've now restricted) to more freely change routes... one that probably suffers higher maintenance costs over subway cars... and one that might be more difficult to electrify.

I think running buses in subway tunnels is just someone's creative idea to attempt to sabotage having reliable mass transit so that ridership doesn't get too high and they can argue that the whole program should be cut a few years down the line.