Metro, planning theorist and PlanPhilly head Harris Steinberg envisions burying I-95 along the Delaware Avenue waterfront from Washington to Spring Garden. Pragmatist and Deputy Mayor Rina Cutler (if you're reading this, I'm still looking for a job), on the other hand, has espoused a "rebuild-in-place" position whereby existing infrastructure gets renewed as need be, but no new far-reaching plan is espoused.
The forest I fear both are missing for the trees is that I-95 doesn't need to be there in the first place. The purpose for which it was originally designed--transporting goods to and fro along an active, pier-studded waterfront extending all the way from League Island to Torresdale--lapsed long ago with the containerization revolution and the implementation of the modern practice of concentrating port facilities in large terminal complexes. In terms of its original design concerns, it is obsolete. (This is true of most waterfront expressways.)
In terms of interstate travel, it is duplicative. There are no fewer than four different major routes between Baltimore and New York through the Philadelphia metropolitan area (to wit: I-476-PA TPK-NJ TPK, I-95, I-295, and the NJ TPK) via limited-access highways; travel time differences on three of them are, at best, limited. Yes, I-95 is a spinal Interstate--but it's because of this reason it should be routed around the city center instead of through it (as it is in D.C., Baltimore, and Boston).
I-5 in Seattle is also spinal (running essentially between Vancouver and Tijuana), but there is a strong debate there whether to bury it or eliminate it*. Arguments in favor of elimination are strong and applicable to the Philadelphia situation. Rerouting I-95 via either the Blue Route, I-295, or the NJ TPK (or, as I have suggested before, a combination of the latter two) would shift through traffic out onto the beltways instead of the urban core, thereby making closure and eventual (urban) redevelopment of the section from Oregon to Allegheny** feasible. Once you've closed the expressway, you've also gained redevelopment parcels, some which even come with ready-made basements, which in the long run will eventually destroy and reurbanize the gap between city and waterfront. This plan is also much cheaper than even Cutler's, since it would mostly constitute re-signing and bureaucratic transactions leading to the sale of developable parcels.
Current traffic along I-95 is split between through traffic (although the directness of the NJ TPK makes this not as much as you'd think, particularly southbound) and local traffic originating or terminating in Center City. Local traffic needs can be met, in the urban core, via good surface arteries, and I-95 happens to parallel one such: Delaware Avenue, which has been re-signed Columbus Boulevard through Center City and South Philadelphia. This road has plenty of capacity--especially when augmented by the grid--to pick up 95's slack.
To summarize: closure of I-95 through Center City Philadelphia would be (a) cheap and (b) productive in terms of releasing developable parcels in revitalized areas, as well as in terms of reuniting city and waterfront. The structure can also be reused for other purposes (one possibility that springs to mind, congestion issues aside, is an El spur) or simply auctioned off, the easement destroyed. Land clearance does not even have to happen prior to conveyance--some plans may well find the existence of the deck a useful amenity for e.g. the provision of a roof, or postindustrial-grit ambiance, or even for parking. In fact, infrastructure clearance in certain segments--i.e., where I-95 is already sunken--is counterproductive in that it destroys existing amenitization, thereby undermining the land value, and reducing the amount recoverable through conveyance. Through traffic needs can be met by rerouting I-95, and local traffic, via Delaware Avenue (Columbus Boulevard), which parallels this easement most of the way along this corridor. In this way, redevelopment of infrastructure past can be used to meet urban needs of the future.
* Presumably by rerouting it along current I-405 (eliminating that designation), extending I-90 to Tukwila, and re-designating the resulting spur from Lake Union to Lynnwood I-305.
** The Oregon terminus accesses Packer Terminal and Allegheny the Richmond Terminal. North of Allegheny the expressway would be re-designated as a spur road (possibly I-395? I-695? I-995?) to the juncture with current I-276, which may or may not be so designated in the new numbering scheme.