Many of those old Hifi systems had speaker with 100db at 1 watt 1 m. Many stereo amps were also rated at 25 watts where as today you would see 100 today. I see why they had to be so efficient, the amps and technology couldn't do 100 watts or at least not for the price.
So as amps got more powerful why did they start making speakers less efficient? With more power it seems like you would get more bass and mid bass with todays speakers having big heavy magnets and matching voice coils. Was this the compromise to becoming less effective? Seems like the speaker companies would have to know the amp companies were doing this.
How did they get bass out of 12" cones with those little tiny magnets or did they not?
Heres a related stereo technique:
My knowledge is limited to car stereos but I always wanted the loudness of a 15" with the accuracy and punchiness of a 10" sub. I figured out how to do this: Get a high end 15" sub that can handle a lot of watts 500 in my case. The speakers needed a 1 cu ft of air behind them and the speaker holds .5 cu feet in front or took up .5cu feet in back : So I took a .5cu foot box and mounted the speaker upside don so you saw the back of the speaker. I wired it so that the speaker fired into the box. For flattest frequency response you want a sealed box as ported have a peak frequency and bandpass have even greater peak. I then got a high end D class amp that did a lot of current that was higher in wattage then the speaker around 800 watts. The result was a super loud but more importantly really clean bass from 20-110hz. You can go to 110 because of the small box size and also still have the loudness at 20 because the amp is so big. It also looked cool and when listening at normal volume had tight sharp response. most subs won't do the really low frequency unless it turned up loud. This probably wasn't possible with those high efficiency speakers. Two 1 Farrad caps along with a 160+ amp alternator were needed for this to work.
Instead of chit chatting about subjective matter, lets get back to the original question.
That really depends what kind of speaker you're looking for.
With PA speakers (sound-reinforcement) you can still find drivers with high efficiency.
90-100dB /W is not uncommon.
The biggest difference is basically optimisation in "the motor" (the magnet system).
The response of a loudspeaker is a big pile of compromises that need to be made.
In the old days amplifier power was VERY limited and very inefficient (maybe couple of hundred of Watts at most with a PPP KT88 with basically no efficiency, lol)
To compromise for that speakers were designed to fit in a bigger volume.
Since you can compromise electrical power with acoustic volume.
Also, you need to take the word "designed" with a grain of salt.
Knowledge back than wasn't as good as today, mostly because FEM analyses and optimisations.
There is actually also another side of this story.
When developing a loudspeaker you roughly have a compromise between efficiency, lowest frequency (Fs, resonant frequency), total bandwith (so what will your highest frequency will be), distortion, maxSPL levels and needed acoustic volume.
Since power isn't such an issue anymore, most manufactures give up efficiency to get a lower Fs plus the speakers can be used within a lower volume.
(people don't want enormous speakers anymore).
The magnet does play part in this, but it is definitely not always telling you the whole story.
Also the material of the magnet really matter, ferrite, neodymium and in the old days they used alnico.
But that gives yet another compromise in terms of thermal heat capacity and such (you can never win
)
In some cases people want maximum portability, so this means a lot of (extreme) compromises need to be made.
Bluetooth speakers are an example of this (sorry, but I don't see how people can bring a 18" horn to the beach?)
So in the end it's all about what people really want and need in a certain environment.
Unfortunately the marketing department had a good go with that and that's why there is so much misconception and myths around audio.