Of course, some people are bored. Many who post on the forums about this topic have entry dates on the forum from 7+ years ago. It might be boring because, well, they have 7+ years of experience with the battle system. I'm certain that I've progressed to a point where most dungeon content is no longer difficult, regardless of the job I am currently playing.
But this poster isn't really talking about dungeon instances. They're complaining because of Raid content (as is anyone else who mentioned Nier in a post). You can tell that's their main point of interest because they mention "casual and QoL changes, Mythic PF and other bs" -- what's missing from this list of reasons for player loss? Raid content.
To be clear, as someone who spent 10 years playing that game, WoW lost out because they backfilled the major story line through books. WoW lost out because their "content" devolved into a daily grind with little or no story at all. WoW lost out because our characters were never important in the grand scheme of things. We, as characters, never did anything worthy of notice by major characters in the game. We, as characters, did not defeat the end-boss of a raid or dungeon, or even major story line battle. Some entitled NPC did. In a cutscene. Or in a book explaining how Garrosh, for example, escaped judgement and moved to another version of Draenor, while in-game we were not informed about anything other than the assumed incompetence of our leaders because, somehow, Garrosh moved to another Draenor.
As for "Classic" WoW, I played through it when it was new material in a game called World of Warcraft, without that "classic" label. Was it as difficult? Not particularly. But then I believe this poster is thinking about raids again, rather than dungeon instances.
There is one very very important Mod that Real Vanilla WoW never had that is available for Classic: Deadly Boss Mods. How difficult can it be to raid when every boss move is noted by an active mod? Talk about braindead.
As an aside: "achievements" were part of the Wrath pre-release, not available for the first 4 years of the game. How "classic" can it be if it incorporates mechanisms that were never available when the game was Real?