PLA isn't really a counter example since PLA is a singleplayer only game. You can't even do online battles in it, let alone have a reward structure for doing it.
It's also probably the lowest effort Battle Facility type thing, it uses an arena that was basically just decoration in the base game and it's just event coding, which is probably a bit time consuming but also doesn't really have unique assets.
Being able to rebattle/battle important trainers has been more of a trend in recent generations than Battle Facilities, ironically.
Let's break this down:
(Note, this is adding to my distaste of Serebii because apparently it doesn't have a resource for the Champion Defense battles lol)
Sun and Moon has Champion Title Defense which IMO started the trend, though B2W2 is arguably the real creator of the concept of course. In it you can rebattle most important trainers along with other trainers that are miscellaneous, but often with unique designs or important in some way:
-Faba
-Gladion
-Hapu
-Hau
-Kukui
-Molayne
-Plumeria
-Ryuki
-Sophocles
-Tristan (aka the youngster joey guy lol)
USUM takes away Molayne and Faba to add in Lusamine and Guzma. This along with the Battle Tree IMO marks a recent trend in the franchise where they seem to really value giving you a way to rebattle important trainers, and not just Gym Leader rematches.
Sword and Shield continues the trend when, in DLC2, you're given the Galarian Star Tournament where you will play in double battles in pairs with plenty of story relevant trainers, if not all of them:
View attachment 687902
And then in Scarlet/Violet we get the Academy Ace Tournament, clearly a successor to this concept. You can fight
-Arven
-Jacq
-Dendra
-Geeta
-Hassel
-Miriam
-Raifort
-Saguaro
-Salvatore
-Tyme
-Penny
-Nemona
-Clavell
Which is basically every base game character trainer that matters IIRC. And then you get the Indigo Disk rematch capability with the DLC, where you can rematch basically everyone.
Legends Arceus' training grounds also lets you rematch a bunch of trainers, though not everyone.
So IMO it's clear that one of their focuses with post-game content nowadays is letting you rematch the characters rather than playing against NPCs, probably partially because they tend to be putting more emphasis on the characters in general.
I don't know if I'm misreading this part, but Blueberry Points are basically entirely designed to be grinded online in co-op.
Like I actually quit doing the Blueberry Dex because I started a year after people stopped caring about the DLC, because I calculated it'd be something like 30 hours to grind the points needed solo, and that'd be tedious. The game also continuously encourages you to do it online with friends, and people talk about how it is a really fun co-op experience.
BP is like very explicitly designed for multiplayer tbh.
I spent like 300 hours in SWSH even though I thought the games were mid, a lot because of the Dmax Raids. It's addicting to get on and see what rare high stats Pokemon I could get, and to have friends ping me that they found a shiny we could all grab.
Dynamax Adventures in the DLC also took up a considerable amount of my gametime with it being really fun to grind with friends tbh.
The incentive for dens is simple: Good Pokemon. All of them are guaranteed to have pretty good stats, they were the only way to get HAs on launch, and they also gave plenty of other rewards.
Den grinding was very common before the DLCs. I'd say the DLCs took away a lot of the reward by adding Ability Patches, making HAs much easier to obtain.
On top of just giving good Pokemon, it was also basically a built-in dupe, with all of your friends getting too mooch off of your find and catch their own. I remember hopping on several times in December 2019 when friends told me they got good Ditto raids, for example.
This is the kind of thing of why I don't care for "Getting competitive Pokemon should take 5 seconds and have no work in it" because a lot of the endgame stuff is about getting good Pokemon in recent gens, almost like an MMO that doesn't blow up the Switch.
I still think it'd be nice to have battle facilities, I just don't agree that SV and SWSH don't have thought out endgames.