hi,
As far as I am aware, they have never gone away..
Here is the definition of the word comeback: "A return to formerly enjoyed status or prosperity" and in that context they are not as popular as they once were and my post has merit.
Understand that the word comeback does not infer that the concern in question has "gone away" as you suggest above.
As discussed I said "magnetic amplifiers might make a comeback" and yes they were more popular by default as the alternatives were fewer!
Wiki says and I concur:
In later years, improved vacuum tubes, followed by the transistor, would come into play for the production and regulation of high-power AC and radio waves. Recently, however, a resurgence in the interest and applications of the magnetic amplifier is resulting in new uses for this venerable technology.
At one time, magnetic amplifiers were considered as an alternative to vacuum tubes because of their rugged construction, high reliability, and lower working voltages. Consequently, vacuum tubes had a definite lifespan due to heater filaments that eventually burned out over time, or a loss of vacuum resulting in potentially-destructive arc-overs within the tube.
Magnetic amplifiers were used early on to control large, high-power alternators by turning them on and off for telegraphy or to vary the signal for voice modulation. However, the alternator's frequency limits were rather low to where a frequency multiplier had to be utilized to generate higher radio frequencies than the alternator was capable of producing. Even so, early magnetic amplifiers incorporating powdered-iron cores were incapable of producing radio frequencies above approximately 200 kHz. Other core materials, such as ferrite cores and oil-filled transformers, would have to be developed to allow the amplifier to produce higher frequencies.
**broken link removed**)