Titanium is just about a perfect material for firearms, at least the actions and perhaps barrels too but it is as you stated very hard to machine and consequently difficult to cut or form rifling. Besides it's corrosion resistance it posseses a quality of being able to withstand millions of stress cycles without fatigue. It is as strong as most gun steel and plenty strong for the pressures found in firearms. Casting or any hot working of it is more difficult because it must be done in an inert environment, above 1000 degrees F it rapidly absorbs oxygen out of the atmosphere ruining it. I fabricate all kinds of parts out of titanium, welded and machined and it is a wonder metal in the right application. One other down side to titanium that is a factor in firearms is that threaded fasteners tend to gall so you need to use a dissimallar material screw or bolt and some type of anti-seize on the threads.
Will titanium replace steel, no way, steel is the most usefull low cost strong as hell and readily worked stuff on the planet. Steel is still the miracle metal just like when with the invention of the Bessemer furnace we first were able to make it.
Mike C