If true, then it begs the question; why were U.S. police departments overwhelming using 158 gr RML for 40 or 50 years?
IGNORANCE
What is commonly known these days by gun owners with internet access (or gun rags in years past) was not commonly known back at the turn of the 20th century. The way to get MORE POWER was to move up to a larger diameter (.32 to .38 to .44) cartridge with commensurately more mass (100gr to 158gr to 240gr) or to a cartridge with more velocity (.38 Short Colt to .38 Long Colt to .38 Special to .357mag). Not everybody was reading Elmer Keith and casting LSWC.
INERTIA
Never underestimate the power of inertia. What did the last guy in my position buy last time? I'll buy more of that...
AVAILABILITY
"We play both kinds of music here: country AND western." "We produce ammo in large quantities for volume discount in two varieties: target wadcutter and LRN."
ENVIRONMENT
Which changed in 1965 with the "crime wave." The social controls of Jim Crow were abolished combined with a new and hostile attitude toward LEOs resulted in MANY more perp-police shootouts. Toss in greater drug use and what was getting it done pre-1965 was no longer getting it done...or 158gr LRN never got it done all that well, but now there were more examples coming faster demonstrating its inadequacy.
158gr LRN PERFORMANCE
There are mathematical models, anecdotes, data gleaned from shootouts and all that. Those generally point toward 158gr LRN being a poorer performer relative to other options.
What really cemented it in my mind was an article in a gun magazine touting LRN as a great small game-getter. The argument was that centerfire cartridges with SWC or HP pills tended to turn small game (rabbits, squirrels, etc.) inside out and destroyed the meat. OTOH, LRN caused little more disruption than .22LR LRN or plated LRN. But, yeah, I experimented with LSWC and LSWCHP vs rabbits and squirrels. Head shots were mandatory, as thoracic cavity shots left a mess I would not want to eat. LRN allowed for boiler room shots, anchored small game fine, and left much omre meat edible.
I think a lot of the knock on the 158 RNL is propaganda put out by the ammunition companies in order to sell us more expensive hollowpoint ammunition. They are continually getting us to jump through hoops. Look what is happening with the 40/9mm. All of a sudden the 40 is not required and you've just got to buy the super-duper 9mm ammuntion. It's happening with the guns too. You can't get much selling a 40 today. I bet in a few years the situation will reverse. So it goes.
MARKETING
No argument, marketing is a big deal.
9x19mm NEW HOTNESS
The "better bullet technology" story is only part of the answer. Yes, many ammo makers now produce ammo that performs IAW FBI standards in gel. To include 9x19.
But I think the real answer is political. Affirmative action for women and other folks not traditionally in the gun culture results in students/employees that can not qualify with a more robust cartridge like .40S&W, .45ACP, etc. 9x19 is easier to teach a noob or someone marginal to shoot for qual. Think of that Somali cop in Minnesota who shot the gal who called 911. I be dollars to donuts that dude barely qualified or did not qualify on many tasks, but was pushed through to graduation of LEO school.