Plymouth died of neglect that started around the 1975-1981 timeframe. That said, sales were high through 1990, and then they fell off a cliff.
1. Plymouth got Cricket later than Colt (but it did get Arrow). Plymouth stopped getting its own wheelbase in the late '70s, it became a badge engineered Dodge. Plymouth didn't get the fancy 2.2 or Shelby packages on Horizon/TC3, Sundance, Caravelle, no turbo Neon, Acclaim, Breeze was late and no V6, AWD Laser was late. Prowler was just too little too late.
2. Plymouth got no equivalent of the '75 Chager, Mirada, 400, Lancer, Dynasty, Monaco, Intrepid, Daytona, Avenger/Stratus Coupe, Stealth and Viper. Remember Plymouth was the brand of Sport Satellite/Road Runner/Superbird/Sebring/GTX, Sport Fury, and Valiant/Barracuda/Duster/Scamp.
3. Eagle sucked up a lot of money and some models that would otherwise have gone to Plymouth. Premier and Vision should have been Plymouth Fury. Talon intentionally sucked all of the wind out of Laser. Even Summit/Vista stole Colt/Vista sales. This coincides with Plymouth sales falling off a cliff. I guess the thought was that Chrysler/Plymouth dealers could deal without Plymouth sales better than Jeep/Eagle dealers could deal without car sales. They moved base models of LeBaron/Cirrus/Sebring sedan,T&C and Concorde downmarket to pick up Plymouth sales.
After that there really wasn't anything left for Daimler to do except pull the plug. Chrysler management, especially management in the '90s killed off Plymouth.
If they are going to do a Plymouth revival, there is no better time than now. Here are some ways to do it:
Platform: Chrysler STLA Large, Dodge STLA Large-Medium, Plymouth STLA Small Plymouth becomes the brand of value and small cars.
Powertrain: Chrysler BEV, Dodge BEV-PHEV, Plymouth HEV-ICE. Plymouth becomes the no plug in value brand.