People expressing their personal opinion that Cataclysm heroics weren’t relatively hard and acting like that’s actually the consensus view are being ridiculous. Ghostcrawler wrote a blog post saying they intentionally made heroic dungeons more difficult than they were in WotLK. I still remember what the forums were like back then because it was the first time I got suspended (for making fun of people who were upset about the difficulty—I was a different person back then)
If you were to ask Ghostcrawler what went wrong with Cata, he’d say that the dev team wrongly assumed that if they made content slightly harder, players would rise to the challenge, when in reality they’re more likely to just quit. In fact I’m pretty sure he did say something along those lines on Twitter