When air is under enough pressure, it reverts to a liquid. In a steam system, it's not the air (if there is any inside) that reverts to a liquid; it's the steam itself. Steam is water in gaseous form.
"In liquid form, it takes up less space, which removes pressure." It does not remove pressure because pressure is the only thing keeping it in liquid form. Remove the pressure, and you get steam (with constant energy above water's saturation temperature).
"It takes a significant amount of pressure to cause water to revert to a solid, a pressure far in excess of most equipment." Not really, considering modern car radiators allow water to remain a liquid even at temperatures above 212 degrees. A radiator cap is only designed to hold, typically, around 12-16 psi. The pressure will depend on how much energy the system has.
"This generally makes water-based pressure more dangerous than steam-based." Not really. As soon as the water comes out of the pressurized system, it becomes steam in atmospheric pressure. There are other factors that come into play that make them both equally as dangerous.
Also, I wouldn't want to operate a solid system (water under pressure) without some form of steam volume. Steam is compressible, it gives you an expansion volume. Water is not. A small temperature change in that solid system results in a drastic pressure change, enough to lift a relief or damage the system.