Like most things in life answer isnt black and white. Pokemon anime both improved and declined in some aspects in my honest opinion.
I think biggest improvement which can be noticed in anime is in battles, becoming more detailed with more effort being out into it. Rivals and pokemon are by far better being treated nowdays as well.Back in day only few of them had their personalities fleshed out gaining some growth,but today show is more focused on treating group pokemon like individuals gaining some notable growth.
Characters are also being developed more with anime not being anymore centralized around Ash and his journey. Granted in BW formula was back to Ash as only main star again but Iris and Cilan have more defined goals, amd writers learned how giving rivals to all companions helps to move their story a lot.
However i have to admit,that in older days pokemon series had that fresh aspect to them with journey being more about exploring world of pokemon,exploration of friendship between traveling companions with anime having more emotion to it.
For instance "Bye,Bye Butterfree", "Pikachu Goodbye!","Gotta Catch YA Later" or when Ash died in first movie etc are hard to ignore and stay indifferent.
Plots used to be more creative imo and anime sticking less to games, brought that air of unpredictability to it with viewers never knowing what to expect out of it. If something is taken from games its naturally much easier to predict i what way things are going to play out,unlike its case with something which is only product of writers imagination.
Chemistry between main group was zealous,endearing and lively being highly intriguing. Ash,Misty and Brock brought conflict, humor, tension and promoted friendship being clearly attached to each other which scored huge popularity.
Imo they had best dynamics out of all main cast combinations in pokemon.
So in technical sense pokemon improved without doubt,but in emotional and comedy aspect quality dropped a bit with previous sagas being more creative in that aspect.
I also feel how constant cast replacing kinda damaged anime integrity and stable story in longer run as far as changed made in anime over the years are concerned. Now dont get me wrong in moderation everything is good, but just like you can add too much condiment in dish your preparing you can also exaggerate with constant cast replacing. Taking away from consistent story, continuity and more substantial development of characters reaching in end nothing but bunch of unfunished, underdeveloped and never to be seen again companions which could offer much more if returned refreshed, updated and improved through new exciting storylines and ways to see build up of their lifes and plots. Which is what is happening now until pokemon reach 50th + or so random new companion overcrowding place so much that it would be impossible to show them together even for finale. Making their existence and any started storyline made about them, influence and role they played in Ash life and series storyline pointless in first place if they will be abandoned, ditched aside like they never mattered becoming forgotten.
What I like about the anime is that they're prone to accept long-held suggestions by fans. Like they brought Brock back after Orange Islands, they actually made Team Rocket more serious in BW, etc.
Not always though. Some of most consistently talked and complained about things in show over the years like Ash never winning league with writers expanding on hmaster career. Misty coming back to main cast with many being peeved about her unfinished and untold story (like her water goal, ambitions, unbexplored past)missing fluid dynamic she brought, May getting specials about her contest adventures in Johto etc. were never took in consideration from writers.
Making me suspect how writers in general donpt pay much attention to what fans want. Though there seemingly exist exceptions to this rule like you brought up.