The bridge-in-a-weekend replacement scheme has proven very successful for conventional bridges: highway, railroad, road crossing, and river crossing. Massachusetts in particular has made good use of it to put a dent in its maint backlog, including on the Fairmount Line commuter rail.
But there is a biiiiiiiiiiig difference between that prefab I-84 bridge and a movable span over a major navigable river. For one, staging space. You have to assemble each span of the bridge whole and be able to set up cranes and jacks to slide it into place. That takes an enormous amount of side room, which is usually available around suburban highways but not so much around densely-packed downtowns and especially densely-packed downtown waterfronts. There's zero room (esp. on the west shore) on the sides of the Norwalk River to stage anything for a replacement, much less a replacement containing all the movable-span machinery. Second, you'd never be able to swap out a replacement deck onto the same pilings here like you would a cookie-cutter 1950's highway bridge. That doesn't work when the bridge is quite this old and would need major remediation work on the supports. This is why the replacements of all remaining NEC movable spans call for the new bridges to be constructed adjacent to the old instead of on top of the old one rail deck at a time...all-new pilings needed as well. They've pretty much replaced all the bridges whose new decks can feasibly be plopped on the same supports...the really hard ones are the only ones left to go.
And finally, the bridge is unreliable in large part because it's an outdated swing design on a river that no longer has water traffic fitting the profile of a swing's specialty. They want to replace it with a fast-opening lift span that has redundancy for lifts one pair of tracks at a time. Lifts work great over navigable rivers where there isn't a need for particularly tall ships...they go up and down fast and can go up to adjustable height instead of all-or-none, don't have a lot of moving parts and aren't as failure-prone or maintenance-heavy as swings or bascules, are (relatively) cost-efficient to build compared to other types, the decks can be much heavier and more durable to maintain without requiring oversize counterweights, and they allow for a wide water channel with least risk of barge strikes because all the parts get stuffed vertical in the towers. For the Norwalk River this is ideal: it still has lots of short barge traffic, but only a little bit of recreational traffic north of the harbor. A lift would significantly widen the channel and let the boats get in and out faster when it's open, in addition to being superior for rail traffic. If they've got to do a permanent fix...it's best to do it as a lift instead of another swing.
BTW...modern swings work best when you've got heavier traffic of tall--but not wide or heavy--boats that need to keep an orderly pace to "the side of the road" when passing each other. Mystic River and Colt's Cove, New London, have those 1984-construction swings on the NEC right by large recreational docks. Lot of taller-mast sailboats that go in and out of the coves near the same time of day and have to form a queue on either side of the bridge to pass each other simultaneously during openings, but not a lot of openings on off- boating hours, offseason, or with anything time-sensitive on a set schedule. They're small boats, not barges, so the bridge-strike risk is minimal and the passing quenes can be allowed to line up before an opening. And because of the strictly rec. nature of the boating if the swing is having mechanical trouble and has to stay in the closed position as fail-safe while being serviced, the rec. boat owners are expected to suck it up and go elsewhere. Whereas you can't really tell a commercial barge, ferry, or the military to go bring their pickup truck to the nearest dock and take their boat out of the water if the swing is temporarily out-of-service.
Everywhere else on the Sound the preference is for lifts (like the Thames River bridge was retrofitted into) or generic bascules (CT River bridge replacement), because those can be built with the span-per-track redundancy and fit the profile of the relatively modest water traffic of the navigable rivers on the NEC. And that's why--when they've only got one shot to do it--they go for purging the swings everywhere except for those 2 specific recent-construction ones in New London County that fit a very specific boat traffic niche.