I disagree, kinda; for me, the degradation began with XY, in 2013.
Why? Well, look at Gen 5 (BW and BW2) for a moment - we had finally gotten constantly-animated sprites, which gave newfound life to each and every species - and the sprites were lively and unique, perfectly fitting each species' general character and personality. BW2 added onto this by making every Trainer also animated, thus giving each one some personality too.
As for gameplay, it was perfect - there hadn't been any nerfs up to this point, only additions on top of additions on top of a complete reworking of a fun core. New abilities added, new held items added, new species added, Rotation and Triple Battles added, all great.
In terms of in-game activities, BW2 shined in that regard - Pokéstar Studios, Black Tower/White Treehollow, Challenge Mode, PWT, and keeping BW's Battle Subway. So much to do, in just one game! Not to mention the optional Memory Link, adding optional scenes if you linked your BW2 save to a BW save, and the decent amount of postgame content. Absolutely wonderful.
...Then XY happened, and threw practically all of that into the trash. No more Memory Link, no more Challenge Mode, no more PWT - just the lame-ass overly-feminine Battle Maison, after an extremely short and boring postgame. No more lively sprite-based animations; Pokémon models barely moved in comparison, and due to the god-awful Sky Battles the models for many Flying-types were left floating extremely awkwardly in mid-air (look at Tropius, Skarmory, Swellow - even Salamence). Animated Trainer sprites were mostly replaced by static art in Gen 6, which was a severe downgrade.
XY also started the trend of implementing nerfs with each new Generation, which hadn't happened before as they weren't needed - sure, permanent weather was a problem, but why were special moves nerfed? Stuff like Thunderbolt and Ice Beam (95 base power), Fire Blast and Hydro Pump (120), and Leaf Storm and Draco Meteor (14) were never once considered 'overpowered', so why nerf them to 90, 110 and 130? Huh?
Also, here, Steel stupidly lost its Ghost and Dark resistances - at the same time Knock Off got its massive buff, setting the stage for Ghost's dominance in Gen 8 after Pursuit's removal.
The main adventure of XY was boring as heck too, and Route 13 or something - the wasteland area with the wind, the power plant, and the wild Pokémon that rapidly chase the player down? Fuuuck that area; the wind needlessly slowed traversal down, and the wild Pokémon were unaffected by Repels, making it even more of a chore to explore - and most Pokémon there also had a chance of having Arena Trap too, forcing you to actually fight instead of running if you didn't have a Flying-type or a Levitate user up front. UGH.
Every single Trainer was weak as balls due to rarely having a full team of six or full movesets, and while you could rematch the Elite 4 and Champion they wouldn't have stronger teams, as that was relegated to the RNG-reliant Battle Chateau (where they weren't guaranteed to appear, especially if you still had a low rank). Lumiose City was confusing to navigate, and home to a literally game-ruining bug before patches (and now that the eShop's gone, permanently so if you didn't download said patches).
It also didn't help that most of the actually-useful TMs were relegated to Kiloude City, in the postgame, leaving the player with weak TMs during the main adventure - and I could go on a whole separate rant about how SHIT the Friend Safari was.
There was almost nothing enjoyable about XY, and it started the degradation that Let's Go, Sword/Shield, and Scarlet/Violet would all hasten in their own ways - at the very least XY still retained all moves and species, even if tarnished due to needless nerfs, and at the very least XY introduced the awesome Mega Evolutions.
