I think its already declining to a degree, but its exactly why issues like getting more women into Magic: The Gathering (an ongoing saga if you follow MTG) are important:
When I was a teenager and in my 20s, my primary social activity was gaming, online and in person. And I don't think it is an exaggeration to suggest that, at that time, many of those things were over 90% male. How the hell are you going to learn how to conduct yourself around women, present yourself, approach them, etc if you never do? You can't.
As gaming becomes more mainstream and women partake in more of it (and they are still minorities but not nearly to the degree they were at those times), it will enable these young men to have more typical developmental paths.
The typical 'incel', 'r9k', etc type of young man is a product of growing up in an environment where, intentionally or not, his hobbies were almost universally female-free, and so he was never given the opportunity to learn how to behave around them.
The other problem is that, like you mention, some of the things TRP advocates for work in either the short term or incidently. Getting in shape is clearly effective. Confidence is effective.
And it isn't entirely untrue that 'assholishness' is ineffective. What governs attraction and what governs relationship success aren't necessarily the same. Let me addendum my story with how I overcame all these problems and learned to interact with women:
I had already learned the conversational/confiding interaction from my questionable HS endeavors. I played a lot of games but eventually got into a fairly serious roleplaying game - I played that game for years. It had a number of actual female players in a fairly small community (150-200 people).
My first character was a paladin and ended up the game-wide leader of Knights and paladins. It was where I began to learn leadership/administrative skills, because I was responsible for a lot of stuff. Like any position of power, I got flirted with occaisionally in or out of character, but nothing significant.
I left the game for a little while because I was working, came back, and made a new character who ended up eventually being the mirror: he was a manipulative, (lawful) evil knight who became the leader of the Knights of Evil and was a major player among all the badguy factions. He gave approximately no fucks and was quite powerful. I modeled his Machiavellian behavior on Cao Cao from Romance of the Three Kingdoms.
That character (and sometimes me straight up out of game) got hit on constantly. I mean constantly. I mean daily, and every 2-3 days out of character (via MSN chat, aim, etc) from female players I had already known in my previous capacity. The amount of internet (or even real) sexual offers I received was insane.
This gave me a lot of experience interacting with women in flirty or outright sexual conversations and did a lot for my confidence and understanding of how to approach women sexually and what would turn them on. I got to mess around with or have fairly intimate sexual conversations with a couple dozen women from around the world of approximately my age, sometimes for a few weeks running at a time before we would get bored. Some of them would call me, others would Skype, yet others would send selfies, etc.
And absolutely zero of that happened when I was playing a good guy. When I played a polite, charming, manipulative evil shit, approximatley 70% of the female playerbase of that game tried to mess around with me in some capacity or another.
It was absolutely surreal. I'm not claiming some of those people didn't have issues (many did) or that they were seekign healthy relationships (they werent, they were probably also depressed/sad or seeking attention), but they didnt' seek that attention from me until I played a character who didn't seem to care what they thought or did.
Obviously I have learned more since, but that experience replaced a lot of the HS and Collegiate young adult sexual activity that I didn't do in a typical way, and allowed me to learn about interacting like an adult so that I can have a good relationship with a woman. I've known my girlfriend for almost 10 years and been seriously involved with her for 5 or so now, and I rarely directly apply those experiences, but they built the foundation that I needed.
The primary purpose of ages 18-23 is to try to do adult things a lot and mess up and learn how. That includes relationships, sex, living alone, paying your bills, being personally accountable for your attendance and time, etc.
Skipping those rarely works out.