(quote) I guess really everyone once upon a time thought "Well, if 3000ft is good enough for aerobatics it should be good enough for stalls then, let's go with that!" and so it became an un-written rule (/quote)
That's probably close to the truth of the matter.
Keep in mind that climbing to above 3000 ft in order to recover by 3000 ft is a good money spinner for both the flying instructor and the flying school. Especially on a hot summer's day when the rate of climb is low. Even the RAA "require" stalling in a ultra light like a Jabiru be conducted to recover by 3000 ft. That is blatant stealing money from the student especially as LSA types not only hardly stall they just waffle and recover in less than 50 feet. Again a nice money spinner.
CASA rightly state that a stall is not an aerobatic manoeuvre. Interestingly, some gliders have a wing drop at the stall yet it is quite normal for glider pilots to carry out a stall and recovery at 1000 ft and they don't even have a prop slipstream to aid recovery.
The recovery by 3000 ft should be dependant on aircraft type. For example, it would be most unwise even for an experienced pilot to conduct a powered landing configuration stall and recovery in a DC3 under 5000 ft. This because of the strong possibility of a vicious wing drop on that type. . Remember certification standards of those wartime types were less stringent than now. It can lead to an incipient spin in a DC3. Been there-done that on countless occasions as an instructor on the type.
Someone mentioned one operator had a company rule saying stall recovery to be made by 3000 ft if solo but 1500 ft if dual. Presumably talking about Cessna and similar types. Well, assuming the solo student has been signed and certified as competent to recover from stalls before first solo, if he wasn't competent to recover from stalls within stated height loss, then the instructor would not have sent him on his first solo. But to split the height between 3000 ft solo and 1500 ft dual is illogical. Either the student is competent or he isn't.
The long held 3000 ft stalling minimum practice height has been a flying school/instructor con for decades. But it makes money and that is the main reason why flying schools and their instructors still trot out varying reasons for its retention. Isn't it quite a coincidence that it is also the minimum height for aerobatics recovery?