I doubt they'll be vanishing. The RAM Cummins pickups are very popular, and generally have a good reputation, especially the engines. If RAM chooses to stop putting diesel engines in their 2500/3500 series pickups, I think the market share for the 2500/3500 line will collapse very quickly, and the 1500 line will follow just behind them. Nor do I expect RAM will shift to Ford, GM, Detroit, or International engines. Probably not FIAT engines either. They would take a beating for doing it. Would CAT or John Deere choose to start producing small pickup engines?
We could see some changes in the Cummins offerings. The 5.0L Cummins puttered out. The 5.9 is from yesteryear. And now the 6.7 might not be quite what they want. Will we see another Cummins V8?
The question that is hard to predict is what will happen with the electric vehicles. RAM EVs are coming. So far none of the manufactures have entered the heavy duty EV pickup market, although there are a few local delivery trucks available, and now a couple of long haul trucks. Chevy has announced a Silverado EV heavy duty towing WT model, but it is still vaporware. I'd anticipate that we'll see more and more EVs replacing the 150/1500 lines of pickups while the diesels will remain king of the 2500/3500/4500 series vehicles for at least another 2 decades. Greener Diesel?