One could argue that Pokemon games have gotten easier over time because that's the general trend of video games overall, in which early releases from years ago seemed much tougher than their modern counterparts. In this specific case, some believe that Pokemon games have been more focused on changing the visual style to be more appealing with its 3D models rather than doubling down on gameplay mechanics and difficulty. A prime example of this logic is in Pokemon Sun and Moon, which feature plenty of cutscenes and a diverse region with lots of typing combinations and amazing Pokemon, but with gameplay often seen as weaker than earlier titles.
That Pokemon games were getting easier with Pokemon Let's Go, Pikachu! and Pokemon Let's Go, Eevee! was not surprising, considering those titles were conceived to be introductions to the series for new fans and younger audiences. This is good in-and-of itself because players might be looking for a more relaxed and carefree experience away from the competitive scene, and others might be drawn into the series because of easier starting points. Other entries in the series have been dealing with difficulty scaling in a different way.
Some are especially upset with Pokemon Brilliant Diamond and Shining Pearl being easier than the original Gen 4 releases. For example, experience gain is universal for all Pokemon on a team, regardless of their presence in battle or not, and this is not an optional feature. Things can also get easier in the main story and Gyms because the Grand Underground features critters that are a much higher level than those players can normally run into on the surface, making things more imbalanced.
What makes Brilliant Diamond and Shining Pearl even easier in the eyes of many is that they feature the same affection system as other modern games. While affection and friendship have been a great part of the series since Generation 2, recent Pokemon games elevate that concept to new levels, making some Pokemon resist knock-outs and powerful moves, or land critical hits more often because they love their Trainer. This can be game-breaking in Pokemon Brilliant Diamond and Shining Pearl, because players who face the Elite Four and the Champion are likely already over-levelled, and then difficult battles can become trivial if their Pokemon survive against all odds.
Pokemon Sword and Shield were also criticized for being too easy. Some players felt getting plenty of Rare Candy after each Raid trivialized battle experience, and the main story suffers from not having many puzzles or exploration to do throughout Galar's various routes. Even breeding for competitive Pokemon became a much easier task thanks to all the tools provided to players, from being able to change a Pokemon's Nature to getting several critters with the best EVs possible through Raids. These are all things that are not necessarily bad on their own, and there are pros to having all of this in a Pokemon game. However, when combined they might make the experience look shallow compared to past releases.
Thus there are a few reasons why one could argue that Pokemon games have become easier than they were in the past. However, there is another factor that should be weighed before deciding whether that's good or not: players' experience and aging fans. While it's true that some features introduced with recent Pokemon games have made them easier to a degree, players who went through older titles are likely more mature now, and their experience with video games as a whole has also changed.
Some players might find modern Pokemon games easier because they explored other franchises, and there are plenty of difficult titles out there to the point some extreme ones are very successful precisely because of that difficulty. Pokemon games are also often developed with a theme of coming of age, with younger Trainers proving to be worthy of becoming Champions - though Pokemon Black and White did challenge this to an extent with their more mature narrative. Still, coming back to modern Pokemon titles as an adult can feel different for all these reasons, and the games might seem easier because there is not the same pressure as in the past.
Overall, some aspects of Pokemon games have gotten easier over time, and players have become better at video games or simply come to expect more of them, which changes the perception of modern titles. Still, future Pokemon releases could make things better by allowing players to have more control over what they want in their playthrough and what they don't, possibly featuring a difficulty selection menu. Difficult games can be appealing to some players as much as easier titles can draw in new fans, and that's perfectly fine as long as everyone gets their desired experience out of a playthrough.
Pokemon Brilliant Diamond and Shining Pearl are available now on Nintendo Switch.