A tunnel would be a lot more expensive. A tunnel was considered as an option before the original Key Bridge was built. It was deemed too expensive. A cable stayed bridge may not be the cheapest option, but I think it will be the most cost effective option and give a direct hazmat bypass of Baltimore.
OTOH, the tunnel would not fall down if/when struck by a ship!
Bridge, given this turn of events, turns out to be penny wise and pound foolish, does it not?
Just sayin