The original Yugioh was one of the most interesting anime I ever watched. Once God cards were introduced, anime became much more fun and you could really enjoy and it felt like it had a greater purpose.
Even the game was fun and dueling was very simple and well enjoyed.
Then Yugioh GX came, and it felt like a slight downgrade, and the reason why it felt like a downgrade can only be understood if you know what made original Yugioh so good.
In original Yugioh, Yugi often fights for something much greater, everything, life of his family, of his friends, even his life is often in danger. People use all sorts of dirty tricks in duel and outside of duel to bring him down. However, his friends always support him and support each other. Yugi has to make all kinds of choices, even losing duels only to avoid harming innocents.
Then there is great rivalry between him and Kaiba, but Kaiba isnt the bad guy in the anime, he just wants to defeat Yugi in a duel and always wants to prove his superiority, which makes it more fun.
GX felt like it lacks a lot of that. The design of characters went from "cool" to "lets be funny and look funny".
It lost the serious look and serious story.
Then 5D came, and it became even worse. Duel while racing? Its monster animations, despite being produced years after, somehow looked much worse than original yugioh.
Then all the other ones, zexal, arc v...
The animation just kept getting worse.
Even Yugioh game is becoming worse and worse.
Konami figured out the money making combo: Release cards more powerful than previous, making all previous cards useless and forcing players to buy new cards all the time. Do it over and over.
Of course, if you duel online, you dont have to buy cards, but other problems are still there.
People just copy top decks, forcing you to either use top decks either lose 80% of games.
You spend your time making and understanding one deck, only to have it be made useless by new cards, or destroyed by ban list.
I have bunch of decks from before which are now useless and cant win against top decks. Its just that top decks are on a whole other level. For example, Kashtira deck can obliterate the field, discard opponnent's hand cards, prevent summoning by blocking field zones, summon bunch of high attack monsters where each has a search effect or combo to add cards to hand, has basically 12 cards which act like monster reborn/special summon from deck.
Its not possible for many of the older decks to compete with that. Simply, the engine is too consistent and too good for it to be matched by some deck from 10 years ago.
Yugioh combos are also becoming more like entire chess games.
Some combos now take 30 or more actions being made in one turn.
Compare that to simple old Yugioh where you would make 2 to 3 actions and end turn.