Since you are focusing on dictionaries, Webster defines a sub-genre as one that belongs to a larger genre.

The OP asked about new genres on the level of romance, thriller, and mystery. These are higher level splits.

I think it goes back to the intent of the author. If the book matches the niche the author is aiming at then it is fine. Commercial fiction should include the full range of expression from simple to complex. The lines are different for an author that is deliberately aiming for a non-commercial thing.

Readers will self-sort to the kind of books they like.

Terra Ignota by Ada Palmer

If this is deep and well written than all of space opera counts.

Yes but that also goes back to intent. There is a fashion in certain literary circles to make it so a work has to be picked apart and footnoted to make sense. This is stuff like Ulysses or Finnegan's Wake that are not made to be enjoyed. However, when you get to non-standard form and style while trying to be a book sold for entertainment that it is purple.

Doesn't the True Crime Novel arise out of the Penny Dreadful and murder ballad tradition? We have been telling the true crime story in pamphlets going back to at least as far as Jack the Ripper.

It depends on how high up the category tree you go.

Lord of the Rings for example is fiction, fantasy, adventure, epic

Circe is fiction, fantasy, myth inspired, coming of age. One could also make a case for fiction, fantasy, women's lit.

Most LitRPG is fiction, fantasy, adventure, LitRPG

I hold that everything below the second split is a sub-genre not an independent genre. If you want a further refinement fantasy belongs to Speculative Fiction along with Science Fiction, Horror, and Alternative History.

Yes we can all agree on the first level split of fiction vs Non-fiction. Here the only sticky area is how you separate religion/myth/folk tale.

The second level split most people will agree on is the plot or setting genres.

For setting we have Now in this world, historical in this world, or a speculation about other worlds.

For plot you have mystery, adventure, romance, thriller, drama.

After this level of splitting you start getting an infinite amount of sub-genres.

I think the OP is asking about new second level splits.

It’s a sub-genre.

However, LitPRG isn't a new genre it is just a refashioning of fantasy/science fiction adventure story. It's a new style but not a genre. It's not a primary category it's maybe a third level one like mystery to cozy to old cat lady lead.

No that is right of first sale. If you have a physical object it is yours to with as you please. You have the explicit right to resale it.

The issue is that that right goes away as soon as things go digital as you can’t prove who has the one true paid for copy.

It’s not but it comes out of older marketing trends. It comes from the days of stories serialized in magazines. The magazines needed a constant flow of content and readers started to expect that if author A did a detective story last October he would do another this October. Besides, a lot of the pulp end of all genres have had steady yearly releases for decades. That is how most of commercial publishing works.

Most of Dickens, Sherlock Holmes.

Here is a thing I think you are forgetting, how many series do you actually read all the way to the end anyway? There are a lot of series I have started that I stopped not becasue I ran out of books but because it stopped landing. Are these series bad? No. It's just that readers tend to wonder off. A lot of series lose 40% of sales book to book. So if there is a series you do want to read you should buy it. After all, a lot more series end from lack of sales than author inaction.

It's not growing up so much as sidestepping. The space that YA now fills used to be a part of the adult section. It's just that Potter showed how much money you could make explicitly marketing to teens. Since most American adults read at around the 6th grade level the normal level of complexity doesn't change across age groups. All you are changing is the trope set.

There still is on the adult side. The problem is that the YA side is more geared to simple and blunt. If you are willing to read older books most of epic and adventure fantasy used to be about the teen from nowhere that saved the world.

Try The Vorkosaigan Saga. Start with Warrior's Apprentice. Here we meet a guy as he washes out of the military academy due to breaking both legs during the entrance test. Then he takes over a mercenary company with nothing but wits and 4 helpers.

It’s the curation aspect. I volunteered at my hometown library for 4 years shelving books. After 2 I noticed the kinds of books that were stocked and what never made it through the doors. Used bookstores tend to go deeper on non-mystery/romance/thriller that form the majority of most libraries.

1. Look at the displays. Libraries often are promoting a theme and have some displays out.
2. Look at what is being returned. My local library is small so the to be re-shelved cart is accessible. Since this is a random mix of books you end up looking that things you never would have considered normally.
3. Look at the new arrivals. This again is often a random mix across many genres.

At this point I then start sidestepping. Either I spend some time going deep into an author's back catalog or I see what is shelved nearby.

Also, a lot of imprints develop a certain style. If you notice that a handful of books you liked recently have the same publisher start looking for that publishers mark on books.

I hate to say this but a lot of crap gets published. Look into mysteries, thrillers, and the pulp end of SFF. There is a lot of WTF in a lot of bestselling work.

Just the turn to college age guy going on sexcapede with multiple immortal women including a goddess not to mention general mary sue issues. It's either Rothfuss is amazing and the next book will be someone in the corner calling the storyteller on his BS or it's just the utter male fantasy it appears to be. There has been a lot of talk about which way it will end up being on r/fantasy if you care to search for it.

I used to love the Honorverse. Here was a series where a woman being a fantastic hero is normal. Honor is special due to her skill not due to her gender. Then Weber decides to abandon this fantastic and rare premise to focus on religion. Now we have Honor as extra special because she is a woman. We have a giant side rant over evil fundies against good ‘moderates’.

This is made explicit in Safehold which is good not Protestants against evil not Catholics. We had to make sure it was explicit by making the not Catholics commit the worse excesses of the Spanish Inquisition mixed with echos of the Holocaust.

That is just annoying. I wanted to read popcorn books about space navies not religious rants.

They do eventually. My copy of Lord of the Rings was printed 5 or so years before the movies. It has a soon to be adapted banner on it. I think it took about a decade for the movie covers to die out. However, now they all have Ring of Power stickers.

For a generation. Then the fall resumes as the last of the old world dies. There is no God in these books or anything Tolkien himself published.

It’s still one of the best books in the fantasy genre and one of the few modern classics that I love. I’m still annoyed that this kind of hopeful story is now only found in either plotless fluff, YA, or gamelit books.,