Your idea that the popularity of hentai is based on a dissociation from real sex is an interesting…

Your idea that the popularity of hentai is based on a dissociation from real sex is an interesting one, and intersects with cultural studies work on moe. Specifically, in Japan (where birth rates are falling & virginity rates soaring across the board, even among folks generally considered to be attractive), there’s a small but sizeable contingent of folks who are vocally only into hentai, for somewhat complicated reasons.

Specifically: a serial killer who targeted little girls in the late 1980s got strongly associated with anime fandom in the media after he got caught, which crystallized a kind of defensive form of ideas already going around in fan communities about the kind of parasocial relationships fans establish with characters — who are often young and female. The term of art for this kind of feeling became ‘moe’, a pun on the words for ‘burning’ and ‘growing’, usually characterized as a paternal love for a fictional character whose vulnerabilities come off as endearing. While some folks tried to dissociate themselves from the sexualization of these characters, others leaned in & emphasized that not only was the love of a fictional character different than the love of a human being (and thus subject to different rules — for instance, taboos on power imbalances like age and authority make less sense if there’s no one to be victimized by them) but in fact better (because, for instance, love that’s doomed is more romantic or more pure).

The rise of 2D-only otaku (who definitely exist, but whose visibility is probably vastly inflated as they are confused with other disjoint groups like hikikomori and NEETs) predates the economic crash, and clearly, most Japanese people who don’t have sex don’t identify this way. Instead, it looks like folks who already weren’t having sex produced a justification for it that also lowered the social stigma associated with watching or reading hentai, & everybody else came up with a different justification.

But, the landscape of hentai has changed a lot over time too. I haven’t done the data mining necessary to check this rigorously, but from the perspective of a westerner who has read a lot of hentai doujin over the past 15 years, I see a couple trends in the popularity of particular attributes, which seem to complicate your idea above:

1. Outright western-fantasy and eastern-fantasy settings in hentai have become substantially less common since the mid-90s. This is not accounted for by a drop in fantasy-genre original properties: there are more of those than ever, due to the isekai / narou-kei boom, and completely original hentai with fantasy settings are down too. Instead it seems to be the result of
2. The fantasies are, on the whole, less rapey than they used to be, & to the extent that sexual assault or non-consent is used as a plot point, it’s usually either substantially less violent or is subverted in other ways. Fantasy settings often were used in older hentai to justify rape scenes while skirting censorship rules: a monster can be expected to be cruel and violent, and can also be expected to have phallic appendages that are not technically penises (see for instance La Blue Girl). Tentacle porn has transitioned into being mostly consensual in hentai doujinshi these days, often showing up in books with only female characters and taking the role of living sex toys. Likewise, other fantasy elements still occur, but rather than being a justification for violent assault, they are justifications for other kinds of impossible or impractical acts (where Bible Black used magic to justify the futinarification of a sadistic satan-worshipping female teacher who then screws a student clearly under duress, modern hentai more often use magic to give a penis to one or both members of a lesbian pairing, who use it for consensual sex; gender-bending & animal transformations are used the same way).
3. With the transition away from high-fantasy settings, we instead see a lot of magical-realism: otherwise modern settings in which vampires, succubi, and transforming cat- and fox-girls try and fail to pass as human; where the internet is full of hypnosis apps that really work; where a science experiment spontaneously changes the genders of a whole school. In addition, we see a lot of simple wish fulfillment, mostly associated with idol anime: comics where fans of the show accidentally stumble into an opportunity to have casual sex with a character, or to become the character’s secret girlfriend. (A popular variation not involving an idol: character A accidentally discovers character B performing sex work, and character B is worried that A will think poorly of them, but A praises B for their open sexuality and confesses their long-held crush.)
4. The fantasies also have a lot of negativity turned inward toward the presumed-male audience. Where the more violent hentai of earlier eras was concerned with non-consensually humiliating female characters, hentai that’s concerned with female humiliation these days usually portrays it as part of a consensual S&M relationship. Instead, you start to see a lot of hentai concerned with a male protagonist who sees himself as undesirable (because of his weight, or because of phimosis, or some other physical problem) and is pleasantly surprised that some female character desires him. (A little less often, this is mutual: the two see themselves as undesirable but each other as desirable.) This gets combined with incest, which is very popular: taboos about blood connection & age differences get explored with a foreground of affirmation. I think this points to a more emotionally vulnerable and less aggressive audience — the ‘grass-eating man’, as the stereotype is called in Japan. This is probably a demographic shift, and it’s reflected in content: to the extent that violent assault is portrayed in new hentai manga, it’s usually being perpetrated by old men in positions of power, who are drawn grotesquely.
5. There are also a lot more queer pairings of all varieties, for various audiences. All the trends I mentioned above are reflected outside of straight pairings to varying degrees.
6. Rather than using the freedom of manga to draw fully fantastic worlds and creatures, it has settled on inventing creative ways to show the feelings of the characters (something that is mirrored to some extent in the trends in TV anime, wherein even most fantasy works have settled on an off-brand Dragon Quest setting in order to focus on expressive character animation). For instance, x-ray penetration (wherein a cross-section of the inside of an orifice is shown to indicate normally-invisible state like muscle contraction or ejaculating inside) and agehao (an exaggerated o-face, often combined with symbols written on the pupils of the eyes) are popular. Sometimes, fantasy elements are adopted entirely to support stylistic concerns — for instance, slime girls (shape-shifting humanoids made of gel) are extremely popular because, as gel, they are translucent (providing x-ray penetration) and all of their body area constitutes a potential orifice (opening up new possibilities for foot and armpit fetishists, among others).

Basically, as hentai becomes more popular worldwide, it also gets more grounded in modern japanese life & shifts toward adopting norms of behavior that would be less problematic in the real world, mostly by turning inward and producing introspective, character-based porn. This is probably a good thing: sex-starved people in modernity are also often touch-starved, and the wholesome turn in hentai is therefore probably also fulfilling a need for emotional intimacy.