I have never seen one in pieces, but I believe that the Sturmey Archer gear was effectively an epicyclic gearbox similar to that used in an automatic transmission in a motor vehicle.
In my younger days, I had a bike with a five speed deraileur. At one point I was going to add a double chainwheel at the cranks to make it a 10 speed, but I had such a wide set of ratios at the wheel that the deraileur was already at its limit for the amount of chain it had to take up between bottom and top gears.
Then when I reached 17 years old and I could drive a car, there was no going back to the bike!
Coming back to Vals original point about how many gears do you need?
I would say more than three, but 27 sounds excessive, but then these days things are often done because they can be done relatively cheaply.
Consider a motor car, in the 1950s many cars only had a three speed gearbox, especially the cheaper models, or those with big engines which had lots of low down torque and did not need a very low first gear to get up big hills.
Nowadays, only the cheapest car would have a four speed gearbox, the five speed box is the norm and many quite everyday cars have a six speed gearbox, especially those with a diesel engine which have a relatively narrow rev-range.
And talking of diesel engines, trucks have big diesels with a narrow power band, and lots of speeds in the gearbox to allow them to operate at the most economical engine speed under varying road and load conditions.
Those gearboxes are often automatic or semi-automatic, something which would have been unheard of in the 1950s.
I guess that is progress.
JimB