Yeah, I mean Blizzard has to specifically design around Echo/Limit for RWF - they literally despawned Nzoth on all realms because Limit was going to kill him in an unintended way too early back in 8.3. GMs actually watch their progress live so they can adjust stuff if necessary. SLG was almost certainly just thrown up as an insane block because SD might not have been tweaked yet.
Like it literally takes everything the devs have to stay ahead of these groups. They would clear FF or any other game within a day.
WoW actually has more than 4 difficulty modes now - the Denathrius that Echo and Limit killed is different than the ones Hall of Fame guilds killed which is different than top 500 guilds killed which is different than now. Mythic raiding ends up having like 4 iterations by the end of the patch because the gap between #1 and #500 and #1000 is so incredibly extreme in terms of skill/organization.
Blizzard invests a lot of developer resources is fostering the RWF scene. No other developers are willing to do that. We can like or dislike that approach, but it is very clear that WoW remains the most difficult and most competitive MMO by a huge margin.