^Not saying it's impossible, but it's not easy as 1-2-3 either. I've thought about it a bit, and while this isn't the thread for it, I'll share my conclusions I reached ~last year. I can share sources if desired - they're in some messy list in a text file that I'd have to sort through.
It's still a large and long bridge handling a lot of different loads you have to design for. Baseline bridge replacement unit costs in 2017 for MA were $472/sq ft. Even if you just put a park space above and relocate the roadways, that's a pretty substantial cost. It likely would be a tad higher than the $472/sf considering rising construction costs and increased loads from the parkway (soil is heavy and static, while the roadway loads are dynamic, and everything would probably be asymmetric), say $550-600/sf at an optimistic level (though this is likely calling for a complex construction method -$$$), plus $10/sf for the park (based off the adjusted $/acre for the Greenway). I forget if the reconstruction costs included costs associated with utility relocations/accommodations, but those are definitely going to be present, and will likely add more to your cost. And, sorry to keep growing the list, but doubling the size of the Greenway would mean doubling the maintenance budget, which is already getting a lot of heat for how much it costs and how shallow their pockets are getting. Realistically, who pays for this? It's screaming Public-Private-Partnership, with emphasis on the private part. MassDOT doesn't really get much at all from this (perhaps a loss in maintenance and ops), City of Boston possibly gets increased tax revenue, and developers get much higher RE values.
Could a few real estate developers pull it off? I'd say it's not entirely off the table, but it's not near the top of their list right now. You'd need to be established in a booming market that's ready to hand out loans with increased risk. (I suppose you could to that now with lab space, but who really wants lab space in urban centers? The city is screaming for housing, and I'd consider living next to Greenway 2.0 if I had the cash). The next step (that I never got to) is sketching out a possible layout (there's a sketch somewhere on aB that seemed to work great), calculating SF of new real estate vs sq ft of park and roadway, and narrowing down your construction $/sf, seeing how much you can sell your RE for ($/sf), and compare it to your costs. Long term, the payback can possibly be there for devs and the city (assuming the park is non-profit-operated), and the community and health benefits are great, but that's long term, and very few of us look at the long term with enough care, especially lenders (who run the show) recently...
(Sounds like a good starting point for a thesis project for any prospective arch/civil/ResD students reading, if any).