Speaker magnets

this may help

https://www.tonmeister.ca/wordpress/2014/01/31/bo-tech-how-to-make-a-loudspeaker-driver-a-primer/

Field coils have been around a long time.  They add a lot of weight to motors and generators/alternators.  A 150lb starter of old is 40 lbs at most now. Fixed neo magnet starter motor, the solenoid is almost as big around as the motor.

They say FC speakers are some of the best ever made.  They have to be expensive to make too. Interesting critters!!

Regards

Generally the bigger/higher gauss the magnet has, the better. Yes over time the permanent magnets do get weaker, but they can be reenergized. I still have some Altec and JBL from the 60-70's, great speakers, Alnico magnets. The newer Neodymium magnets are great and very hard to demagnetize, expensive, but a little goes a long way. Look at the gauss rating if specified, helps control the cone movement. 

An FC mag puts heat around the VC something you don't want. It adds needless complexity and for some strange reasons sounds the best powered by a Tungar DC power supply. The older FC designs were overbuilt but I wonder if the heating-cooling cycles an FC produces would affect the longevity of the FC in modern FC transducers. That being said FC, Alnico, rare earth, Iron ferrite mags all can sound fine depends on the overall design of the transducer and imprementations when used in a finished loudspeaker.

There are magnets and then there are magnets.  The older speakers either used Ferrite or Alnico magnets.  The alnico magnets were used in some of the better speakers at the time, like JBL.