Well, to be honest like SoulSilver/HeartGold they only had dared to changed things that does not warp the timeline or story of original games. For example, the expanded PokeDex for both Trainers and Gym Leaders alike, or how Mega/Primal Evolution was introduced, etc.~
Yes, added storyline on how Kyogre/Groudon can Primal evolve is quite big and or even the addition of Delta Episode, but the story of them*OR/AS* just ends up similar to the original games just with more elaboration, much like so on TyranitarTube's reply video about DLC.
The addition and implementation of some of the said changes is to attract new players as usual and or also as an added incentive to older players. For example reintroduction of other region Pokemon and Mega Stones after Cave of Origin arc. While some added changes like numerous legends from other regions is there to support a new added storyline; this time a whole game added with extra legends just to suit the story of the Hoopa. And or to exapand/elaborate on storylines from the original games; an example of this, we finally knew why Steven had to go off. But one can even argue that Mirage Island and Mirage Tower's story is also being elaborated through this.
Adding features only visible in Pokemon Emerald at that time, which has deep relations with R/S will be quite confusing since Pokemon works in a timeline. The changes viewed in HG/SS such as PokeAthlon or two regions in one game was quite big, but it did not change or warp the story orginals. Instead we get elaborations on some storylines such as where did Giovanni left to, etc. Plus, some can even argue that one other reason for Battle Frontier not showing is that in the original Emerald, your first meeting with Brandon mentions of him just coming back from a journey, so going by timeline of the game, R/S timeline is still when he is still out on a journey.
To complain lack of Overworld Pokemon is a bit aloof sounding since there are more Overworld pokemon in OR/AS than in the original games.
If you properly checked, you should know there is a Lvl. 200+ Zigzagoon in the game, which you did not mention.
The concept of Legendary is to attract people. IT DOES suit the story since the person plays as the protagonist of that world. As far as the game is concerned, each game is a copy of the same universe/world, communication between trainers is also seen this way since PC operators in the game; Lanette on the original game *R/S/E* stresses on how technology is improving that Pokemon can now travel in space/time through PC. Going by your idea more Pokemon players will cheat and or quit since not everyone will be good enough to win titles.
The difficulty is to soothe larger population of new players, that is 15 and below, you may not find it difficult since you have learned from past games, but each game always tries to target new players, therefore they have to make it just right for them. Gym Leader badges is there to make it not too easy since an over leveled Pokemon will not follow its trainer. And more prominent characters or more powerful trainers always uses potion of all sorts, being limited at 2 mostly.
The way you put your criticisms only target experienced players. Does a new player/intermediate player even have that much gold even by the time of Pokemon League? Or even that such high leveled Pokemon? The truth is their Pokemon will likely be under-leveled exactly why experienced Pokemon streamers usually tell their viewers and also guide books tell players what's the optimum levels Pokemon should be to successfully defeat a gym. If you are knowledgeable on this, you will realise that a level cap of 10 levels is there for the first gym Roxanne which has pokemon of Lvl 13, etc. going above this will mean you Pokemon will not follow you anymore.
However, I do have an issue about AI being stubborn on not switching. And I do agree that having a 'Challenge Mode' like Pokemon B/W2 would be awesome.
But for people complaining about how raising Pokemon became easier, EXP. SHARE CAN BE SWITCHED OFF.
If anyone have a view on why Japan mostly have all events I'll happily address it by copy and pasting one of my comments in another thread.