Besides, nothing is stopping Game Freak from adding it as DLC in the next games. It understandably takes a fair amount of effort, so put a price tag on that payoff.
Or, alternatively, making it an app. GF doesn't even need to be the one who makes it. Fans can't be the only ones who would not just the Battle Frontier (and other various Battle Facility-like features or alternate way of battlign) but also Pokemon Contests (+ Musicals) & Pokeathlon (and other competitive minigames they've had over the years). Heck, a more expansive Pokemon Camp app would probably make gangbusters (which, thinking about it, was sort of what the RS Box and Pokemon Ranch was except not as interactive...). I'm sure there are plenty of developers who would bite at the opportunity to make apps like those.
Not only do they not have to put it into the game, but also they can simply update the app when a new gen comes along to just allow those new Pokemon, Moves, Abilities, etc.. While they may eventually need a new version of the apps, I can see these apps lasting for several generations if supported the entire time.
If the frontier does happen, I could see it using dmax mechanics like the dmax adventures does. In fact, dmax adventures reminds me of those small stations in emerald (that replaced contest areas) where you'd play by picking pokemon from a selection and swapping from the opposite team's (I think this one was in slateport).
Maybe, though probably not. Dynamax Adventure is a very different thing from what Battle Facilites do. Dynamax certainly would have been included, though I don't think in any special way as it is used in the Battle Tower (or Mega Evo & Z-Moves was used in the previous gens), just yet another tool for the player to use. Also Dynamax likely isn't going to be carried over to the next gen so...
Also you're thinking of the
Battle Tents.
It's pretty much necessary for every Battle Frontier to have it, so casuals who don't want to train battle ready Pokémon can still use the Battle Frontier.
Except for the Gen IV Battle Frontier. Battle Tents pretty much existed to introduce the concept to the franchise in addition to those playing the game. By the time of Platinum players knew what the Battle Frontier was so no need for the tents, which is a bit sad as they were a neat idea to give players a "taste" of the Battle Frontier before it fully opened up post game.
On the topic of important facilities for a frontier from the perspective of bringing in new players, I'd probably add something like the Pike or Pyramid in addition to the Tower (for obvious reasons) and the Factory (for the reason you mentioned). Dungeon crawling with some HP management matches the style of the ingame more than the other facilities, and is something the PvP scene can't provide. Castle in gen 4 comes close with the resource management, but doesn't have much in the way of actual dungeon crawling.
Don't forgot the Black Tower & White Treehollow from BW2. Not Battle Facilities sure, but certainly a resource managing dungeon crawler.
On the other hand, people gotta put their money where their mouth is.
Money I don't think is a scenario in this case. Emerald, Platinum, and HGSS made plenty of money, but GF still ultimately decided to cut the Battle Frontier as the post game location.
Now their reasoning in Gen V could have simply been wanting to do something different + being they were purposely going different directions (yet by the time of BW2 decided to also mixing in paying homages to the past) they did the Pokemon World Tournament bringing back all the Gym Leaders & Champions. Sure they were normal Pokemon battles, but they were able to create different tournaments like going by region & Type. Heck, the Battle Subway itself averted certain Battle Tower conventions like being available during the main game & have one of it's line take you to a location you couldn't otherwise (though I wished they did more of that, like had the different lines actually take you to different cities once you unlocked them).
For the Gens after that? Well Gen VI had no third version, ORAS they were oddly hyperfocused on making it just Ruby & Sapphire remakes they excluded the additions Emerald added to the games (including Gym Leader rematches), by the time they made USUM the post game story concept had taken hold and GF found those more interesting to do, and now Gen VIII had the DLC where they focus was on making more interesting Wild Areas cause that what they were more interested in doing (and likely weren't able to do what they wanted to do with it cause of time constraints).
By the time of Gen IV I think GF had their fill of the Battle Frontier so moved onto wherever their next whimsey took them.
Maybe this is unpopular, but I feel like the oras frontier tease was less of a "fuck you" and more of a "we didn't forget, we just couldn't make it"
And who was asking them to push out the games before they could put in a Battle Frontier or it wasn't worth delaying it to add it in? It has nothing to do with money, it has nothing to do with time, it's not even about what their target playerbase considers wanted content (and remember we're not the target playerbase). This has always been about what GF were interested in doing and wanting to do it within a certain amount of time.
Still, expect plenty of people mad with Game Freak when they see the GC is closed in the inevitable Sinnoh remake.
Or they'll just make it into an arcade like in Let's Go (maybe even add in some mini arcade games we could play this time...).