Part of why the green Prudential Building is now available is because HD Supply downsized and then moved to the Avenues a few months ago. Prudential also vacated their space a few years ago. I believe the major tenant remaining in that building is the U.S. Army Corps of Engineers.
Haskell's property was originally master planned to have, as I recall, two more buildings, each coming off as perpendicular to each end of the existing riverfront building. Kind of surprised they wouldn't push to build one or both of those if they wanted more space. Maybe they are concerned about resilience to the river flooding although I think their building is reasonably elevated. They may have a parking issue too given any expansion requirements.
The building appears to be well taken care of so I would think it would amount to Class A office if put back on the market for that purpose. Would be great for an existing company in town in the burbs to relocate to.
FYI, Haskell's prior building in the 1970's and earlier was the multistory building, now painted black with a Jax/Florida mural around the top, along side I-95 at the west foot of the Fuller Warren Bridge.