If there was no benefit to LTRs, our species would have evolved away from them a long time ago, but we haven't.
Yes, the under-40 crowd doesn't enter LTRs as much as previous generations, but I blame that on technology rather than some sudden realization that LTRs are a bad idea. Technology has led to the rise of the red-pill, black-pill, and wannabe alpha-bro communities that try to make men feel macho for staying single. In reality they're just justifying being emotionally lazy and misogynistic.
Technology has also allowed women to juggle several suitors, hookups, simps, cucks, sugar daddies, and everything in between, reducing the desire for an LTR.
We have a big problem with dating/relationships in the west and many other countries are screwed as well such as China.
Generally, most women are going after the top 20% of men? And maybe more like top 5% of men. Kind of always been this way. Especially to sleep with him, then find a cuck to marry and divorce him later and take child support and alimony. Nature is cruel, most men are invisible to most women. If the women doesn't like the guy more than the guy likes the woman, I think the relationship is doomed. She most be the adorer and the guy must be the adored. Women have much higher standards and understandably.
Here's a few of the issues. It might look like I'm blaming women, but men are at fault too. I have no answers, just a few points.
Porn
Videogames
Simps
Social Media - Women get a ton of attention from simps, over-inflating their sense of worth, and also compare their life to other women that have a "higher status" guy. Also, women are getting flown out to rich men's houses all over the world. So now a man isn't competing with other men in the local area, he's competing with men from all over the world.
Women earning more money - I'm not against it, great for women, but now women are more independent and don't rely on men for financial security. Allso due to society don't need a man for physical protection. Imagine just 100 years ago, the average woman depended on a man for protection, so she had no choice but to "settle".
Hypergamy
Feminism
Promiscuous young women that sleep with lots of men, they become alpha-widowed and can never be satisfied with a man on their level after that, unable to pair bond. The more men she sleeps with, the more likely she is to get divorced if she chooses to marry someday. They think just because a man that's more attractive than them sleeps with them, that they are on his "level". Major delusion.
Marriage laws
Bay Area - A tougher dating market for "average" men. Plus women are a little more feminist/masculine/independent/promiscuous here.
Men not earning enough money. Ironically women want to earn as much as men but they still want a man that earns more than they do.