Oh also, on the Double Battles talk: I think Double Battles is inherently the objectively superior battle mode for singleplayer, despite me vastly playing singles in competitive. Someone who has successfully nuzlocked Radical Red, Pokemon Reborn, etc., games that are 99% singles at some of the highest difficulty
This is mostly from a game designer perspective so allow me to nerd for a second; doesn't it strike most of you as odd that basically every more traditional JRPG (especially turn-based RPG) combat system has multiple members that can attack and be attacked in the same turn?
It's for a good reason, it's to make the game easier to balance and more complex/interesting at the same time. Because in Pokemon, if you can 1v1 the opposing Pokemon by clicking the same move twice, the game feels boring. You are just clicking the move that says super effective and winning.
You make that double, triple, etc. and you have more complex turns that demand the player's attention. Now they have an Abomasnow and a Blastoise, and if your Charizard just clicks Flamethrower, it will die. Maybe you are okay with that as you have a second Pokemon to defeat the Blastoise afterwards, but generally players see losing a Pokemon as inherently them being punished, and the game being harder.
Something I noticed when Legends Arceus was new was that people tended to find a Pokemon fainting to be more indictive of difficulty being present, with the battle system favoring trading back and forth so heavily. I think this is something I can trace within myself as well. Even if I win the battle, having even two Pokemon faint makes me feel like the battle was tougher, even if the strategy crafted was still fairly simple.
In a Singles scenario, you have your Cinderace click Flame Kick or whatever the fuck, the Abomasnow dies. The game prompts you to switch out of the incoming Blastoise, or if you are set you just see it come in. You switch out to your Grass-Type and win that, and cycle again.
Having more Pokemon be present on the field at the same time creates more visceral feelings as Pokemon are inherently harder to take down cleanly (ie. with none of your Pokemon sustaining major damage). Double targeting, having more strategies as the games are more and more balanced around them, and it's also easier to create difficult challenges.
As of right now the best they can do without forcing players to essentially make competitive teams is go "wow! that trainer had a focus sash! now you click ember again." But with Double Battles you can make compositions that are not hard for an average composition to defeat, while still having more complexity.
So, this begs the question, why is singles the standard?
Nerd out moment two: So, essentially, Pokemon combat at the earliest stages was fairly simple. It was going to be somewhat akin to an auto battler in complexity, and Game Freak experimented with ideas where you couldn't see the enemy HP and had to make a guess from dialogue or a sprite. They didn't want to do standard combat at all, and it wasn't.
As the games were made, how skills were made became more in line with what you'd expect, HP would be there, yadda-yadda. Originally, PVP was going to be cut from the games, as it was one of the last major features not in before ship date. Nintendo mandated it had to be there. They were going to make PVP more simple to make it easier to code, but ultimately just got it done in time (hence, desyncs, bugs, etc.)
So basically, Pokemon PVP being singles was not necessarily because it was the best idea, but more of a necessity. Besides, with Gameboy RAM displaying 4(or more) of those pretty big Pokemon sprites at the same time would have been very, very hard to do.
Obviously the other media at the time took after it, and singles, one-on-one combat became the norm and what people think of in Pokemon. Another reason I think it stuck is that one-on-one fights are simply more appealing to make people care about their critters more. When your Charizard wins? It's... your Charizard winning. There wasn't some second Pokemon next to it taking some of the spotlight, either.
In my opinion, a fully double battles mainline game would be great. Colo/XD are games I do not enjoy, but that is not because of the double battles nature, in fact I think it makes them more fun (outside of the slooow pacing, those animations are still slow as fuck)
Temtem has kinda gone down the shitter by its last update, but the 30 hours I played of it showed to me that singleplayer double battles can be fucking awesome. Pokemon just doing a slight hybrid sucks. Go all in or don't do it at all IMO, as others have said, the teambuilding gets wonky.