EXP All fundamentally isn't a bad concept, it's actually a pretty standard aspect of RPGs nowadays. It doesn't inherently make the game easier, it just makes it less tedious. Tedium isn't a good form of difficulty.
The problem is that the Pokémon games nowadays, for the most part, still don't really offer any particularly challenging or interesting boss fights that help balance out how quickly you can level and other things like affection and super mechanics. Max Raids and Tera Raids seem to be a multiplayer way of trying to do this, and SM and USUM did try this with the Totems, but for the most part, those are very few and far between. The Legends games are actually totally fine even with permanent EXP All in them because of the way you can gain EXP throughout the game being fairly limited and there are actually compelling boss fights, not to mention their respective battle systems actually make boss fights much more difficult and in some cases engaging. Especially Nobles and Rogues.
The standard Pokemon battle is largely 1v1 with both sides on equal footing at a technical level besides level, and in the early games that was a valid form of a boss fight since the Gyms were mini-dungeons and gauntlets and the Gym Leader was a gauntlet of Pokemon in sequence before the ace, counting on wearing down your Pokemon before you make it to said Gym Leader, much less the ace. And simply making them a higher level than everyone else worked. Now, that...really doesn't work as well anymore when a level advantage is quite easy to obtain.
Most turn based RPGs and even action RPGS (like the FF7 remakes) have a particular boss design that's quite rare in Pokemon itself, especially since they count on having multiple party members being on the field at a time against a boss that's disproportionately powerful on their own compared to the party, which actually encourages good strategy on top of doing more than just spam powerful attacks ad infinitum. Using enemy debuff skills and party buff skills, coordinating strategy, and vice versa. Actually that's another thing. There's virtually no reason to use debuff moves and even party buff moves in battles in-game these days. It's tradition for Pokemon to have Leer and Growl early on, and later on there are moves like Swords Dance, Iron Defense, and whatnot, but there's no good reason to use them in-game in most boss fights. There isn't even a reason to use status conditions like Poison, Paralysis, and Burn. Look at the likes of Octopath, Tales of, Trails, Persona, etc. Heck, Persona 5 and Metaphor: ReFantazio both have party-wide EXP All mechanics in them and they're still fun (and in the latter's case, challenging). Metaphor is a good deal harder, but both have a battle system where generally in normal battles it's four party members against multiple enemies, and boss fights are particularly strong and have a unique strategy apiece. Atlus games even have super mechanic skills a la Z-Moves, Dynamax, and Tera, but things like Magatsuhi Skills in Shin Megami Tensei V require a gauge to be filled and aren't an easy win ticket. Among the Atlus stuff I talked about, Persona is on the easier end, especially with the 3 and 4 remakes, but y'know what it is? It's fun. The battles are just super fun to do, like knocking down enemies, getting those All-Out Attacks in, and sometimes getting those Showtimes, Theurgy Attacks, and Prime Time Attacks (when P4R is out that is) when you're able to use them is fun af. Pokemon is easy in a way that's just not very compelling or cathartic in comparison. Steamrolling enemies substantially weaker than you ad infinitum gets boring fast.
Like, I'm not saying Pokemon should be exactly like the other turn-based RPGs, but it could stand to take some notes from them. Other turn-based RPGs like Octopath, Trails, and most Atlus stuff, and you have a better idea of how a turn-based RPG can be fun and compelling. Pokemon makes the player disproportionately more powerful but doesn't offer any challenging or engaging mechanics or fights in its classic battle system to justify it...for the most part.
