There is no world where integrating an engine completely foreign to all other aspects of the vehicle is more cost effective than keeping what they have. Just integrating parts alone into the system would be monumentally expensive.
You're not reading the article and comments. There is no chance of getting the 5.7 back because the tooling has been converted for the hurricane. They're not going to open a new factory for this when it can get shutdown in 4 years again.
The fact that cummins, pentastar, 3 different hemis (5.7, 6.4, 6.2 supercharged), 2 different hurricanes (SO, HO), ecodiesel etc all get dropped in these rams shows that integrating another v8 is not "monumentally expensive". I have no idea where you got that idea, it's not true in the slightest.
It's strange that you've had so much trouble with 87. I think I've ran one tank of 91, with no discernable difference. I've run 87, 88, 89, and on several occasions E30 (94) and I've never had any knock or noticeable difference in power. I don't tow super often but when I do it's usually kind of heavy, as I've been in the 20k pound neighborhood a number of times and even then, never an issue with 87. My brother's ram has the 6.4, tows all the time, and I know for a fact he only runs 87.
It's not just me. Others have complained about it on this forum, or logged it directly from their ECU. In fact, the Ram manual at one point even said something along the lines "a small amount of ping is normal". You don't put that in the manual if guys aren't hearing it.
The uncomfortable truth is that many guys:
A) can't hear it (ears aren't good enough anymore)
B) won't hear it (radio on, talking etc)
C) don't know what to listen for or what it sounds like
D) aren't in the same conditions (humidity, heat, load on the engine, gas quality etc)
I've had one of my brothers not hear a thing in my truck when I clearly did, and neither did my mechanic. It's not a terribly loud sound, it can easily get lost in the noise of the truck or your conversations. You have to know what it sounds like, and listen for it. I've put a video here on the forum from another truck on a dyno, demonstrating the ping, even trying to isolate the sound as much as possible and many guys didn't hear it either.
The 6.4 in the 2500 has a very conservative tune. It would not surprise me in the slightest if there is no pinging under load in that truck.