Loudness - Why has the industry stopped producing amplifiers with this feature any longer?

I haven’t owned a single piece of gear for either home or car audio that doesn’t have a "loudness" feature (it is sometime cleverly renamed) ; and I never will. A true variable Fletcher-Munson is where its at. I like the fatness/punch and warmth it provides at low volumes.
And for what it’s worth - a lot of "purist" are not really purists anyway. How many are running Dirac, or Audyssey, or ARC ? Shouldn’t they be physical treating their room rather than using processing to flatten their response? Room correction processing is just the new/hip thing - so they are ok I guess.... I say use it all - what ever gets you to the experience that puts a grin on your face and that warm fuzzy feeling from head to toe when your system is singing just the way you like it......

I use the loudness on a McIntosh tube tuner, for me it gives the music depth and body, fills out the sound, something is missing without it. 

There are some preamps out there with bass & treble adjustments on the remote. Emotiva makes one.
My old Luxman preamp had both a loudness button and bass & treble controls.
I end up doing a lot of low-volume listening with my Maggies....not a good combination...but bass & treble controls make it enjoyable. 
Have you considered a Schitt Loki equalizer?
https://www.schiit.com/products/loki

I have always been a huge fan of the Loudness Switch for the reason the OP describes. I find a way to incorporate or replicate in all my systems.

Below are my solutions to add "Loudness" capability to my multiple systems:

• System 1: 70’s Vintage Technics Rig w/Reel-to-Reel with "Tape Loops" (x2) and NO factory Loudness switch, oddly = I run an external Technics Parametric EQ via the tape loop "Set to replicate the Fletcher-Munson curve and correct for my room". When I want this enabled, I flip the switch to monitor Tape 2 where it resides. :-)
  
  
• System 2: McIntosh C52 based. I purposely sought out this Pre-Amp because of its EQ, which toggles on/off via front switch. Like above, I replicate the F-M Curve and a hint of Room Correction. I purposely bought this Pre-amp for this explicit purpose and late night listening.
  
  
• System 3: Schitt Loki EQ. Just like above, I use this in my Office system with powered monitors to emulate the Loudness curve.
  
  
• System 4: Computer - Amarra and/or iTunes. I just do it in Software. iTunes actually has a per-programmed "Loudness" setting.
  

Call me old school, but give me this (eq) functionality/capability over using e$oteric cables ’for tone control’ any day. If I want brighter, darker, or tighter sound, I turn a few knobs to my liking.  If a recording sucks and FLAT isn't cutting it, turn a few knobs.  :-)

Bring back the "Tape Loop"!!!!

You could try one of these. It also has a defeat switch for when your audiophile friends visit:

http://www.deliciousdecibels.com/the-delicious-one.html