See I don't agree with that. I wasn't in a "top group" (a good group, but not quite top), as well as knowing some groups that were definitely not top groups that did clear A3 and A4. For a pug? Absolutely, those DPS checks were tight and probably unreachable, but you really didn't have to push your class to the limit to beat them. If anything, they were tight enough to mean you couldn't have a death, but crazy numbers weren't required. Like you mention pushing the 3rd leg before the 2nd set of adds. That wasn't a strict requirement to hit enrage, it just made the fight a lot easier and gave more cushion for enrage. That was more of a preferred strategy thing than a requirement.
But as far as the raid scene dying, it was the midcore groups that died out. I was in a midcore group myself and knew quite a few other midcore groups in the fight. The number one thing that stressed people out and didn't want to keep going wasn't that they couldn't hit the DPS checks, it was that someone would always mess something up and wipe them. I spent some time on Gilgamesh as well, and whenever you'd meet someone who had gone there in hopes for a better group, their former group never broke up because they didn't have the DPS, it was always a mechanics thing. For A3, it was almost always the tornado phase. Groups would spend FOREVER on it because of the general lack of raid awareness throughout the playerbase and progress would be seemingly halted. People don't like spending a long time without making progress, it gets boring. So many groups would hit this wall of no progress and the group would just fall apart.
Again, I admit the DPS checks were a bit higher than in the past, but I truly don't believe that's what made the tier as grueling as it was for so many groups and that it was simply the nature of the mechanics that made it so tough. That's what killed the raid scene.