The new span would be constructed immediately adjacent to the old one, so it would be in full service during construction. They are not, in any way shape or form, considering keeping the old one afterwards.
For one, the new span's middle piers may be so close to the old bridge's swing section that the swing may have to be permanently closed by the time they sink those new piers. OK if it stays closed for 2 years of heavy construction--reasonable accommodation could be made for that--but you can't permanently leave a swing that's blocked from opening by the new bridge's piers. They'd have to start dismantling the swing the very second the new bridge opens.
Second, the approaches--especially on the marshy eastern side--aren't going to be wide enough to feed 2 adjacent bridges. It'll be temporary re-aligning to stage for the new span's construction, temporary re-aligning to shift tracks onto the new span, then mop-up work. But the wetlands don't permit side-by-side approach tracks to 2 adjacent spans. Additional track capacity in the form of the second identical "Portal South" bridge from the ARC plan--should that be needed later--would be constructed several hundred feet south of the current ROW because that's the only way to EIS for parallel spans through the swamp.
Third...the thing is just shot. You don't need to keep it "until" Gateway, because until Gateway the North River Tunnels are the capacity limiter. A tall 90 MPH fixed Portal Bridge span is a huge capacity increase over the current one, with none of the reliability issues. There's no need to maintain a 'reliever' span when that extra traffic has nowhere to go. And for the money it would take--had you still needed it--to stretch its safe lifespan another 10 years you can put a fair-sized down payment on advancing Gateway. It would be counterproductive to do anything with the old one.
More bridge capacity is not going to be needed until well after Gateway. Portal South isn't even a direct dependency on Gateway. They can get by with just the lone replacement bridge for a number of years after Gateway opens before Portal South becomes prudent to independently fund. Portal South plays more into the super-duper 2040 HSR plan and those capacity needs than it does projected 2030 growth of existing NEC traffic patterns.