Structural engineer here who works mostly in bridge inspections and a bit in bridge design. I think someone else mentioned it earlier, but it is highly unlikely that the bridge was designed to withstand being hit (especially at a critical mid-bridge pier) by a huge cargo ship. That would have required the engineers to completely overdesign the bridge which would cost MDOT (at least I assume they own the bridge) A LOT more money and wouldn't have been seen as necessary.
As far as rebuilding, I'm sure the design-bid-build for the new bridge will be fast-tracked as much as they safely can due to the overall traffic nightmare that will be happening. I wouldn't be surprised if they try to build a temp bridge to help get traffic under control in the meantime. But if MDOT has to follow the same rules as MassDOT (the DOT my office is contracted under), they will have to put the project out for bid, which always takes more time. For example, I started a bridge design project back in the fall and we submitted the 25% design in January. When doing the cost estimates for the project, MassDOT told us to assume the start of construction will be 5 years from the beginning of the design phase. That doesn't include the actual construction itself, just the design and bid process. Once again, I'm sure MDOT will move this to the top of the priority list and try to shave time off wherever they safely can, but designing a bridge to cover a span that long will take a decent amount of time.
I'm also just interested in quite a few people saying the bridge being built in the 70's makes it old. This might be a perception thing, but working up here in Massachusetts, while we obviously have newer bridges (big dig and all that), we deal with bridges much older than this one fairly often. So I guess for me seeing 70's really doesn't feel that old when I've inspected some bridges from the 40's and 50's.