I believe it might be true in places with icy/snowy weather. Many people try to reach that magic number on the sign no matter what the conditions are. That means that people driving to the conditions are going far slower than the other people, and as you said - the difference isn’t a good thing, in addition the the fast people being a lot more likely to crash on their own.
For Montana (where I lived at the time), there was no posted day time limit; night time it was 55 MPH. While no posted limit, the law stated something to the effect of "must drive reasonable and prudent for conditions". Conditions here is kind of vague, but is pretty all encompassing: road conditions, weather, traffic, your vehicle, your tires, etc. Bright, sunny, summer day in a Porsche with good tires? At 100MPH, you might have gotten pulled over, your tires checked, and then the officer would send you on your way. Blizzard conditions? Driving 45 might get you cited. And the citation had teeth: it was automatic reckless driving, something like a $500 fine and possibly a suspended/revoked license. Previously, the fines were a joke. Something like $5 for up to 20 MPH over the limit, could be paid in cash and didn't go on your record.
Anyways, "reasonable and prudent" was eventually rule unconstitutional because we can't have nice things. So, out it went, and instead it was replaced with a 75 MPH daytime, 65 (I think? maybe it was 55 still) night time limit. It's now 85 day/65 night MPH as of last summer when I last drove through.
But, the tl;dr is: when the posted limit was put in place, highway fatalities increased (quite dramatically from what I recall). No, I don't have any references to cite, it's just my recollection of events from nearly 30 years ago...
Are you advocating for _increased_ vagueness in our laws?
For an anecdote:
I went to Dallas to watch the eclipse. I got on I-45 to go home to Houston. Traffic was going great, typically about 90MPH in a 75 zone. There were several cars doing 110+ and zooming between lanes while several others were doing a mere 55. The worst were the people doing 55 in the left lane, being passed by people doing 65 in the right lane. The weather was "fine", just cloudy but a pleasant 85F.
Then, just around Richland, there was a sudden massive downpour and visibility went down to about 100 feet. I could feel my front-wheel-drive wheels losing grip for seconds at a time, even at 75.
It was unusual for my vehicle to lose grip at that speed, but I'm used to driving on slippery gravel roads. I also had expected (planned for) it because I had looked at the weather and knew it would be raining by the time I was home. Gotta keep everyone staggered and plenty of distance between each other. So I let off the gas and coast down in speed until traction was regained.
Even so, we didn't have to go very far on the freeway before everyone was 100% stopped for about 45 minutes. The weather was moving northeast (to a good measure of perpendicularity), so the weather will have hit that whole stretch of freeway about the same time. Someone at the "front" hydroplaned and wrecked with someone else around Streetman where I-45 turns west-ward. By the time I made it to the front, all of the emergency vehicles and both damaged vehicles were nowhere in sight -- so at least there's few rubberneckers.
Driving for the _current_ weather is a bad idea, and not enough people will consider what the weather could be in 30 seconds. That's only considering the weather. The fact that's people doing 55+ and people doing double that is also a massive safety issue on a 2-lane freeway.
Even inside of Dallas and Houston, people are RIDICULOUSLY crazy. If you drive for a few hours at speed limit on the freeways, then you will pass dozens of people doing 20 below in the left lane, and also have other dozens of people scoot between you and other cars with mere single-digit feet between you and them. There will be countless instances of other vehicles coming into your lane and immediately braking to avoid hitting the vehicle that was in front of you but now is in front of them. If you're ever in slow or stopped traffic, there will be a half dozen vehicles skipping past the traffic in emergency lanes.
So, I think speed limits aren't the only thing that need fixing. There needs to be much better safety education and enforcement. There's more to my opinion but I doubt it'd be popular -- consider how empathy, greed, and modern timeliness also contribute to these problems.