So, there is an issue that between Gens 1-4 it is actively damaging to use full teams of 6, and the lack of EXP duplication in the form of Gen 6+ EXP share is to blame for that. You can cruise through Pokemon Crystal if you just main Feraligatr, but it becomes a tedious grind if you want a team of 6.
My problem is this was fixed in Gen 5 with EXP scaling and Audino, a Pokemon that would appear on every route and provide massive amounts of EXP. This allowed for a solution without changing how the game is played.
EXP sharing changes how you play the game. The reason I don't like the EXP share, at least as a forced option, is because I don't need to actually train a Pokemon to train it. There was a satisfaction of switching out until a Pokemon could win a fight on it's own, like your climbing a wall together. But now that a Pokemon like Magikarp can just stay until my party until it's ready, the entire point of Magikarp is now just redundant. It's not an infamously weak Pokemon, just a Pokemon slot reserved until it reaches level 20, a level where you likely don't have a full team anyways.
Now a Pokemon can sit for multiple fights and only see use when I need it as coverage.
The issue isn't that the games are easier, Gen 1 is one of the easiest Gens. I beat Red with just a Clefairy. The issue is that Pokemon is smoothing out any kind of potential friction to the point that all games are starting to have a similarly smooth surface. You already get a starter for the most part unique to you in the game world that is engineered to never fall off and be very good for most major fights in the game, access to hidden stats the enemies never get to use until endgame at the earliest, enemies don't even run berries anymore let alone other held items outside of post-game, this is just another "main-character" advantage in a franchise already criticized for storylines where characters quite frequently tell you how special you are.
To be clear, I really like Gen 9, and there are some creative fights in the DLC that do provide a challenge. Additionally, Gen 6 is absurdly easy even without EXP Share. EXP Share is not the breaking point for me enjoying the games, I'm just saying there's a kind of hand-holding nature to these games now that dilutes the value of choices when it comes to team building. Like you know it won't matter outside of what backside you're looking at.
"It's for kids" is a common deflection, but so are Minecraft and Fortnite? They have methods and options to get new, young players into their game loops, but when they want to stop holding your hand they will.
Pokemon doesn't need to be dark souls or anything, but I shouldn't have to use a cheat software to get back an option that use to exist that makes the game somewhat harder.