Bev Draw train traffic vs. openings are not approaching nearly enough of a conflict point that there'd be stratospheric benefit to going through the suffering and disruption of reaching for a fixed structure. A lift would've been a lot better than a swing in absolute terms because of faster movement, higher reliability, and potentially much wider shipping channel underneath simultaneously increasing the boat speed limit through the channel and lowering the duration of the raisings. But ultimately technical hurdles are going to determine if a switch in span type is feasible for the construction downsides, and that's not always the case when the last i's are dotted and t's crossed on the engineering assessment. The NEC Shoreline had swing bridge-for-swing bridge cleanroom replacements done at Shaw Cove and Mystic River during the first-wave 1980's high-speed upgrades because tight confines around the abutting marinas didn't allow for anything different without destructive property-taking from those very busy and economically-significant marinas.
Thinking far future when Purple Line schedules finally get Euro-dense, there's still enough finite limits to demand growth north of Salem that Bev Draw isn't going to become a constriction. Rockport's absolute frequency cap still has its ruling limit set by that branch's two movables, which can't be raised to fixed spans because of surrounding layout. And the development-curtailed nature of the intermediate 'burbs between downtown Beverly and downtown Newburyport means that N'port Branch demand (while substantially outslugging current frequencies) doesn't have a full-on exponential burst in it until you cross the Merrimack on another must-be-movable bridge to bring the NH Seacoast back online. There's still plenty of slack capacity left to tap for incremental service increases to the current terminus.
The most unlimited growth to tap is Salem-south served by Indigo/urban rail and the Blue Line extension, which makes Saugus Draw the bridge replacement that truly matters for eliminating the movable span. All BLX-Lynn concepts, including the ones that go through Point of Pines, opt for doubling-up onto the Eastern Route ROW on the Revere side of the river to pool the most expensive infrastructure onto one 4-track river crossing (i.e. the PoP option would cross the river via the shorter power line/ex-Eastern Revere Beach Branch causeway and merge with the Eastern main before the current bridge instead of shooting straight across next to Route 1A on the old BRB&L trajectory with its own independent full-length span). Which was determined to be more cost-effective and less environmentally disruptive because the Eastern RR did the draw approaches way back when at 4-track width with future expansion in mind, limiting the amount of environmental disruption to widen it. The river's also bereft of much boat traffic because of the ecological disaster of silt runoff in the second half of the 20th century that choked its channels to navigational uselessness for everything except the smallest power boats and occasional barge to GE. A replacement fixed bridge wouldn't need much additional clearance, because you simply can't thread a sailboat out of those sparsely-populated marinas with how narrow the channels have become between sandbars. So if they want to mount some expensive SGR effort, a fixed-span Saugus Draw replacement with 4-track abutments + 2-track deck you can slap a second set of Blue-carrying girders on later would be the present-day overpay that pays itself back when the time comes. Changing Bev Draw into something else slots much more as a "nice to have, but surplus-to-requirement".