Only wealthy people will be able to afford to live in OA-Cities, as they will be very costly to build. 

This argument is partially based on the fallacy that, because large pyramidal buildings are extremely rare, it must be due to pyramids being especially costly to build.  But the rarity of pyramidal buildings has nothing to do with their construction cost which, as argued HERE, and HERE, would probably be less expensive to build than conventional vertical-walled buildings.  However, it is obvious that pyramids would be very “wasteful” of air space in city centres, where small city blocks and sky-high land prices force developers to max-out on rentable floor area with tall slab-sided buildings.      

It is also true that any non-conventional building is going to pose extra technical and engineering problems for the building contractors.  

The Sydney Opera House greatly exceeded its original budget during its 14-year construction (1959-73), largely due to difficulties creating shell-shaped roofs, which had never been attempted before or since.  Had the Opera House contractors gone on to build other shell-shaped buildings, those later ones would have been far less costly because they would have learned from experience. But there will NEVER be another building like it because – unlike pyramids, shell-shaped buildings are wildly impractical.  The design was chosen, via a public contest, from dozens of more practical designs as a means of “putting Sydney on the world map”.  In that it definitely succeeded.    

 Pyramids are the optimum form for very large buildings due to their inherent structural integrity with weight distributed over a much larger base. All other factors being equal, large Pyramidal buildings should cost no more to construct than a conventional building. They could even be cheaper given that a pyramid might require less structural steel and shallower foundations than a conventional vertical-walled building of equivalent size. 

Pyramids are rare, but it has nothing to do with building cost…

From the all-important aspect of financial return on capital, pyramids would be a poor investment in most situations.  Upper floors become successively smaller and, since a minimum space is needed for elevators, emergency stairs and so on, the amount of useable space is much less than in a rectangular building. These are the main reasons why pyramidal buildings are rare…

• UNUSABLE FLOOR SPACE 
  • Sloping walls create awkward, mostly unusable, wedge-shaped recesses where walls meet floor.  These redundant spaces are magnified in small pyramids, like the pyramidal houses some people have built.  With pyramids, bigger is definitely better, as this issue becomes relatively insignificant in a large building like the Doha Sheraton, one of the first of many Pyramid-shaped hotels. (Though few are genuine pyramids).               
• SUCCESSIVELY SMALLER FLOORS 
  • Each succeeding level has a smaller floor area than the one below, so a multi-storey pyramidal office building would have much less rentable floor space than one with vertical walls.  A pyramid with a 67.5 deg slope would have at least 25% less floor area than a box building of the same height.   
• ECONOMIC IMPRACTICALITY –
  • In most built-up areas – especially city centres – sky-high land prices would render a pyramidal building commercially uneconomical, even if a large enough site could be found. The few exceptions are very steep-sided, like the “Trans-America Pyramid” (1972) in San Francisco.  Or “The Shard” (2012) in London.  The incline of these buildings is so steep they should be described as obelisks or spires.  
• CITY BLOCKS ARE TOO SMALL –
  • City centre blocks are far too small for the very large base area (building footprint) needed for a large pyramid.
  • Large brown-field sites occasionally become available in city centres but these are also developed for maximum profit. 
  • Or they come up with a useless white elephant like the Millennium Dome (O2 Arena) 
• LARGE SITES AND CHEAP LAND REQUIRED
  • Suitably large building sites are usually only to be found on city perimeters,
  • or in the countryside where planning regulations restrict new buildings to low-rise 
  • But the countryside is the domain of ferocious nit-picking NIMBY’s who can be counted on to object to anything imaginative, especially if over 3-storeys tall.     

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Despite their aesthetic appeal pyramidal buildings are impractical when floor space maximisation is the over-riding factor.  

Except in Las Vegas where the need to stand out from the crowd over-rides practical issues.