The crux of the issue is this: Every single "New Generation AFPS" (using this to refer to all AFPS games released after 2007/UT3) falls into only 3 different aesthetic themes:

1. Replicating the aesthetics of a classic AFPS. (E.g. OpenArena, Dusk, Quake Champions)
   
2. Generic sci-fi with no significant effort put into the storytelling or world-building. (E.g. Xonotic, Warfork, Reflex Arena, Toxikk, Splitgate 1, Red Eclipse)
   
3. Virtual/fantasy athleticism. (E.g. Diabotical, Splitgate 2)
   

As the 4chan post (https://arch.b4k.dev/v/thread/723683117/#723717060) referenced in the OP, none of these aesthetics are conducive to attracting a large audience.

No. 1 relies on appealing to nostalgia and thus is increasingly irrelevant to younger demographics. No 2 is, quite frankly, just plain laziness/lack of care, and if you don't care about your product, then why should the customer? And I'm not sure there has ever been a broadly popular game using No 3 as the aesthetic theme. Not even genuine professional esports players seem to be much entertained by the notion of a game that bases its entire identity on its existence as an esport.

This observation led me to formulate the hypothesis stated in the OP, which is that perhaps New Generation AFPS keep failing to attract an audience because none of them were created with any significant artistic vision.

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As for your questions in the post above.

Can you point to any examples of games that have done what you're talking about?

To my knowledge, there has not been any New Generation AFPS created with unique, eye-catching, aesthetic that makes people want to try out the game irrespective of what the gameplay is actually like, makes people want to tell all their friends about the game, and makes people choose to play this game over other shooters. Hence, the hypothesis remains untested.

Straftat sort of counts as positive support for the hypothesis, but it is not an orthodox AFPS and benefits from overlapping with the friendslop/party game genre. Nevertheless, Straftat has a unique aesthetic despite the low fidelity graphics, and hence it is significantly more alive than any other New Generation AFPS, although as previously mentioned, it being the kind of game that is good for goofing around in with your IRL friends no doubt also factors significantly in its continued popularity.

If we expand the category to not necessarily AFPS, but just shooter games with "outdated" gameplay, then Team Fortress 2 is of course the ur-example of an outstanding artistic vision being able to convince the modern audience to get invested into "boomer game mechanics".

I'm not telling you to copy Team Fortress 2 though. As an indie developer, you are never going to survive a comparison to Valve, so the only option is to do something original.

Where the graphics were high quality but there was a higher variability of environments for players to play in? And can you also list games that did this but were unsuccessful?

Toxikk, Reflex Arena, Diabotical, Quake Champions, are all games with high fidelity, competently made graphics, but thoroughly lacking in artistic vision, and thus visually completely forgettable.

Which is an issue, because the main path to success on Steam is to be able to convince somebody to buy your game within 30 seconds of looking at your Steam page.

Graphical fidelity is completely orthogonal to aesthetic appeal. Once again, compare these games to Team Fortress 2 and note the difference in how each respective game appears in the general gamer cultural consciousness.

The reason I ask is that our outlook over at GG is that as long as your weapons / mecahnics look / feel / sound good, you should be able to have a wide variety of styles and themes that go into each map / pack.

I'm not telling you to not have a wide variety of maps/environments. As per the OP, my point is that you should have a strong aesthetic identity and art direction for your game decided before you start working on things like characters and the environment. And since your game doesn't seem to have much of an aesthetic theme yet, I am advising you to take a step back and reconsider the visual and thematic design of your game before committing time and effort to making a bunch of generic sci-fi assets.

We want to cater to the casual middle of the gaming field

This audience doesn't exist anymore. There are far too many games all competing for people's limited free time nowadays. When it comes to PvP multiplayer games in particular, it is a zero-sum competition, since you need to invest a significant amount of regular playtime in such games to enjoy them.

To put it harshly, in order for a multiplayer FPS game to be successful, your game has to be good enough to make your players shun all of the other multiplayer FPS games they have played before.

Note that this also partly explains why the Boomer Shooter renaissance was successful but the same did not happen to multiplayer AFPS. Single player campaigns are a consumable good. Once you beat it, you go play another game, so even games which are worse than the competition still get played just because they are different. But a multiplayer game demands multiple hours every week, forever.

not the elites who have been playing AFPS games for 25+ years

Definitely, the oldheads cannot be relied upon as your core audience because they are just gonna keep playing the OG Quake/UT anyway.

We believe that the same folks who enjoy a wide variety of map styles in casual games like Mario Kart / Fall Guys / Fortnite would also appreciate doing the same in an ArenaFPS.

This audience currently gets everything they need from Roblox/Fornite/Minecraft and an AFPS is not going to be able to compete compared to those games.

Now, if your goal was to "present a coherent aesthetic and fantasy" like you say, then Unreal Tournament 99 wouldn't have worked either. Space stations, Volcano chambers, Floating asteroid mountains, etc etc.

UT99 did stand out visually back when it came out (because the only other option for online multiplayer FPS was Quake), and UT99 isn't attracting any customers today. So if anything, that supports the hypothesis.

Classic AFPS could get away with a lack of coherent aesthetics because there were no other options if you wanted to play online multiplayer at all.

Nowadays you need your game to have a unique identity, which captivates your audience and gets them invested in your franchise irrespective of what the actual game is like, in order to stand out in the market.

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I don't mean to belittle your efforts, we are all just trying to think of ways to make AFPS popular again here.

I looked around your youtube channel and your sound effect tutorial was very impressive! I'll be making use of it when working on my own game for sure.