Alrighty then, let’s discuss this without mocking or downvoting it.
Now first of all, I think you’re wrong. I don’t think arena shooters are dying off, I more believe that Halo Infinite sucks really, Really, REALLY bad.
To put more of my own evidence forward before moving in on a few claims you made, Apex Legends has over 100,000 viewers on Twitch right now. Valorant has over 100,000 viewers, and Overwatch has over 20,000 viewers. A quick Google search would say that these are arena shooters (Apex Legends apparently has an Arena mode), so my question would be, if Arena shooters are dying off why are two of the top 5 games on Twitch Arena Shooters?
I’d also like to remind you of a little game called Halo MCC. People went CRAZY for that game when Reach was added, and it stayed popular for years. Why? Because it was fun and people enjoyed it.
The online for my argument really bases on the statistics would say you’re wrong as people are still watching Arena Shooters. I’d love to see some numbers on which were the most popular Xbox Games, I think that’d honestly be quite interesting.
Now to attempt to dissect your evidence.
So you brought up Splitgate, let’s talk about that.
Splitgate was stuck on Season 0 for about 2 years? It was in constant beta to the point that people were bored. Not much was added, and with the promise of Halo Infinite, people dropped off. The constant bots being added to matchmade games made it pretty boring, but the game is still doing moderately well today on PC. To say it died shortly after when the game is still going is just false.
DOOM 2016. This game was incredibly fun, like the Arena was just fantastic and I loved every minute of it, but I’m not too sure what happened to it. I only played it in the Beta with friends and whilst we all love it, I simply didn’t have the funding to buy it. It was still incredible though, so I’d likely say you’re wrong with people saying it was considered a complete waste of time.
Looking at top 10’s is somewhat difficult too. Fortnite is actively updated and is fun to a certain degree. Apex has Arena shooter elements and everyone I speak to claims they only play the Arena mode. Kinda seems like you’ve wrote a positive point for Arena shooters here. GTA V is constantly being updated so no wonder why it’s normally at the top. And yeah, Elden Ring is massive as well. Like duh a new singleplayer game will reach the top, why is that a point against Arena Shooters?
You then advocate for Battle Royale’s with the main points being it makes stories and gives players a rush when playing. I’ve got tons of Arena stories, and the constant rush of a Battle Royale? Mate, I hate them because I spend 20 minutes looking for an enemy, then get sniped out of no where by a guy likely using aim bot xD
Like dude, there’s no rush 
You then say Halo / Arena shooters are bad because you need to put in effort, sweat and often the game punishes you. What about the competitive communities that like putting in effort and getting good at the game? What about games like Halo Reach where you could mess around and not worry about doing well? What about Titanfall where you could just run on walls for the entire game if you wanted to? What about Doom where it didn’t matter what you did, it was incredibly fun either way? What about in Overwatch where you had a close game and clutched it out with a well times ultimate attack?
Arena shooters provide more interesting games in my opinion, and Halo Infinite can be incredibly fun at times.
The reason Infinite is failing is because of awful desync, no new content, a company that doesn’t listen to the playerbase, very similar maps, very few maps, and at launch 4 gamemode (only 3 since btb didn’t work).
Halo MCC is still doing pretty well even without the updates and had a good community there.
Not too much else I can say here.