Erosion of concrete by water is a very complex phenomenon, and most Manhattan skyscrapers use steel girders as their structural frame, not concrete, so it is very difficult to say what a flooded Manhattan in 2140 might look like - how loing has it been flooded for at that time? What efforts were made to protect either steel, or concrete, or both, prior to inundation? How fast are typical currents flowing around the buildings (these tend to be much faster near square corners)? How much abrasive debris is suspended in the water?
I would guess that the steel frames of a typical Manhattan skyscraper would last quite well in salt water at least for a century or so, as they are typically well protected against corrosion, and have a significant safety factor in their design - it wasn't until the 1970s that CAD and a better understanding if materials combined to allow less robust skyscraper designs (such as the WTC towers, which were revolutionary in their minimal use of structural elements, maximizing usable floor space, particularly on the lower levels). The Empire State Building is structurally far stronger, as beam strengths and loadings were less well understood in the 1920s and '30s, so architects put in more structure than was (we now understand) needed; the lower floors (particularly the basement levels) of such buildings have significant areas devoted to columns that hold up the building, leading to a 'cluttered' feel and less usable floor space.
A lot depends on the loads applied to these buildings too - if there was, for example, a sea defence wall or walls built in an attempt to prevent inundation, these structures could significantly reduce the tidal currents in many places; As could debris blocking the streets - reefs of car bodies would likely exist, and could give a lot of protection to the buildings (particularly if deliberately and carefully placed).
I can't see that the continued availability of these buildings after flooding by the ocean would necessarily be a silly idea, certainly not as early as the 2140's; And concrete being damaged by salt water is the least of the issues that would face these structures, not least because they don't have concrete as a major structural component. Most skyscrapers in Manhattan are steel frames, built as a cage surrounding a central shaft (used for elevators and services), with concrete floors poured into steel pans to make each floor level, and either steel or (in more modern or refurbished buildings) aluminium frames holding glass panel curtain walls; or concrete and masonry curtain walls - as the name implies, curtain walls are non-structural and 'hang' from the support structure of the floor edges. Steel framed, light concrete, or masonry curtain walls near and below the waterline would likely be gone in fairly short order (a decade or two at most), but this would only effect the structure of the building in terms of allowing water currents to flow more rapidly around the structural elements.
The existence of steel ships and steel bridges is good evidence that salt-water exposure need not lead to rapid deterioration of large steel beams, particularly if ongoing efforts are made to protect and/or repair them. If completely abandoned and left without any maintenance, then several decades of salt water exposure would likely be needed before any of the buildings collapsed, and most would, I guess, survive rather longer; The really important factor would probably be wave action during storms, so a lot depends on the frequency and severity of such storms. Certainly it is storm activity that is the main driver of general coastal erosion, and man-made structures are not really importantly different from natural seafronts in that respect - in the absence of a sea wall or walls to break up storm driven waves, your buildings will likely fall down because those waves hurl debris against them like a wrecking ball, long before corrosion of the steel elements (whether structural girders of concrete reinforcement) becomes a significant factor.
After a fairly short period of inundation there would likely be sufficient damage that no current structural engineer would sign off on the safety of the buildings; but modern engineers work in a highly risk-averse environment. If a building will probably not fall down this decade, you might well choose to live in it (particularly if no alternatives are readily available) - despite the fact that a 21st century engineer would immediately condemn such a structure.
