Wrath was when the decline had set in. It was when the game stopped growing and plateaued. The game was still mainstream popular up through WoD, when the infamous 6.1 patch (which didn't introduce any new content and whose defining feature was the selfie camera) caused such as humongous subscriber drop that Blizzard became too embarrassed to continue publishing subscriber numbers. Supposedly the player count spiked again with BFA's launch. Shadowlands caused the WoW exodus that Dragonflight hasn't been able to recover. Blizzard has offered four or five 50% discount sales for Dragonflight, which is unprecedented for a current WoW expansion.
Vanilla WoW was built upon community. Vanilla lacked a dungeon finder. If you wanted to do a dungeon, you had to find four other people who fulfilled specific roles and had motivation to do that dungeon. The effort required to assemble that group and reach the dungeon really motivated you to make sure that group succeeded. You were out in the world trying to find comitted individuals to play this difficult content with. "This guy can tank and he's good! I need him!". You created a friends list and formed guilds, and then it scales up from there. You created guilds because you needed way more than 40 people to do these 40 man raids (you needed backup raiders who could replace people, crafters, etc). Because raid bosses only dropped 2 pieces of loot each, raiders went in with the mentality that they were working to make the guild as a whole stronger, even if they didn't get any loot that week. The 40-man raid guild was also big enough that you could ignore the people you didn't like and hang out with the people you did like.
When TBC happened and raids shrunk from 40 man to 25 man, guilds shrunk too. You used to have maybe 80 people in a guild, perhaps two 40 man raids happening simultaneously, down to 30 man guilds. That's not many people. Then WotLK happened and 10 man raids became possibility, so guilds shrank again to becoming 15-20 people. When LFG was added mid-way through WotLK, people who were already doing dungeons had good gear and were already competent, so there were no complaints. Gradually, the competency of the playerbase decreased because you no longer needed to be good to maintain your relationship with your friends who you needed to do dungeons with. You no longer had to invest in friendships. Then raid finder and personal loot was introduced, which meant that a guild was no longer necessary. The whole finding a good guild and becoming dedicated to it and forging lifelong friends and being proud of your guild has gone by the wayside. So now everyone goes into a raid with the mercenary mentality of only thinking about what they are going to get out of it, rather than caring about other people.
Wrath also introduced catchup gear, which meant that you no longer had months of content to do as you progressed through raid tiers. Now the game was only the current patch, and once you beat the current patch, you were done.
There was fun to be had in later expansions. I enjoyed the questing experience, aesthetics, music, and class design of Mists of Pandaria. WoD had a banger soundtrack. Island expeditions in BFA were neat. I like the sheer amount of character customization you can have now with the 2 dozen playable races, 20 years of transmogs, mounts, toys, pets, elixirs, etc. But the game is fundamentally new expansion releases > blow through the 5 hour long questing storyline over the weekend and reach levelcap > do 1. the latest raid over and over for months on end until you get bored, 2. speedrun the same half dozen dungeons over and over again until you get bored, or 3. do the same old battlegrounds over and over again until you get bored
. There is no longevity here. I am letting my WoW sub run out next week since I've done the Dragonflight story, toured the raids, and got the vicious saddle and mythic+ mounts I wanted. There is nothing left for me here until after the next expansion launches in autumn 2024.