[archytech]

  Skyscraper flats ... are at such a high starting price that they might put a downwards pressure on really expensive flats but do nothing for the average person.

False.

Without new units, the occupants of the (unbuilt) new luxury units would simply be driving out the occupants of other (more affordable) spaces in older units.

[the_mitsuhiko]

You left out a crucial part which is "Europe" and there it's undoubtably true in areas where I know how buildings are financied and what the costs are (central Europe).