A inventory is usually easier to get than D, on most flights. I can't remember ever being unable to fly on the date and route that I wanted in A, and very rarely was there a need to request F inventory to be released. This has been true on some high-demand routes (CX trans-pacific, QF11, etc.) and I often change flights on short notice. But I don't try for some of the "blacked-out" routes, for example AA has some JFK-LAX flights that have no A inventory (ever). As far as I can tell BA, CX and QF do not play that game (they do have inventory on every flight). You'll have much more trouble with D availability despite the greater number of seats on the airplane.