Flaw there is using the word "challenging" to describe Dragon Soul.
You killed heroic blackhorn/spine with a 5% nerf, and heroic madness with a 10% nerf. Clearly you are in the upper percentage of WoW players, but to suggest that DS provided no challenge is disingenuous.
It really didn't, though. Almost the entirety of DS was merely numbers checks. If you could do X healing and Y dps, then you won, and those X and Y numbers weren't particularly high in Normal mode and got to be absolute jokes in both Normal and Heroic after the first couple nerfs.
This mentality that any guild can just throw away people who've been in there for years and replace with someone you don't know is mainly a heroic guild mentality. But it's not the reality for a lot of guilds out there. They would rather have the dependable guy who does 60k dps and not the jerk who shows up when he feels like it but can do 120k (mostly by cheesing mechanics, dps'ing adds and ignoring raid leader commands because "he's better than you bads").
In other words, the mentality is "Let's just not improve." You want that mentality to be capable of moving forward when it's impossible because it defeats itself.
Blizzard doesn't want average guilds clearing raids "on a schedule" they want you struggling forever and ever in the hopes that you keep your subscription going. It looks like it isn't working, so they introduced a slight nerf only when 5.2 is out. I don't think Blizzard realizes that people don't like being made to feel like they are bad.
According to you "run under the bubble" is a tough mechanic, so I can't take anything else you're posting here seriously.
You've got 2 choices for model where people aren't made to feel like they're bad. One is to make everything so easy that everyone can do it. The other is to make everything hard and let people go through it as they will. You're apparently against the latter. Do you honestly want the former?
Blizzard is requiring that you bring 10 above average players to be able to have a chance at most of the normal modes and this is the first time they've forced players to do complex dexterity mechanics along with very tight DPS enrage timers.
It's "above average" to run under a bubble, is it? Or run in a circle?
Most encounters are either or until you get to Heroics. That is, they either have "dextrous" mechanics - Vizier, Blade Lord - or shorter rages - Protecters, perhaps Garalon.
I'm not sure how you can call any Normal enrage timer "tight." A tight enrage implies that people playing close to maximum ability would have trouble meeting it. No fight on Normal has that. Maybe if you did Elegon in week 1 when your raid's item level was in the low 460s he had a tight enrage, but people in 480 gear? The enrage is a joke to that gear level if you have more than a faint clue of how to dps.
You recollect incorrectly. T11 was never wholesale nerfed - they did a few spot nerfs, but T11 heroic remained the hardest content in the game until MoP was released. Blizzard pretty much said "all you bads keep wiping in T11, all the goods get to go to Firelands". It was a huge controversy. You surely remember.
Considering you're after a better "normal mode experience," I don't see why it matters that the t11 Heroics weren't nerfed. It's not like you needed Heroic t11 gear to progress in Firelands. The Normal modes were extensively nerfed.
Another problem with this raid tier is that the DPS checks are so tight guilds are being forced to 2-heal to even hope to reach the enrage timer and often 1 of those healers is just not used to that intensity of healing required.
Maybe if those guilds had players that could learn to do a decent percentage of their potential output for the gear they were wearing, they wouldn't have to force their healers to play at maximum capacity.
So your suggestion to "hey if you can't survive the transition, just 3 heal" means they will never hit the enrage timer. If they are forced into 6 DPS, then that means their healers have to be exceptionally above average healers or they die during the high damage phase 2.
Well, yeah. If you're working from the assumption of "It's impossible for my dps to put out halfway decent numbers," then you're going to get screwed.
What you should be doing is asking and helping those players to improve.
Alternatively, fix the loot issue. Stop with the weighted loot nonsense that causes an 80% drop rate item to fall every single week and the 3% drop rate item to never drop. It's old. The gear acquisition rate is so dreadfully slow. The elder charms never come through with gear. The whole thing has gotten progressively worse since ICC where all items had the same item drop rate (and actually felt RANDOM). The current design is not random, it's designed to ensure needless frustration.
My @ss every item in ICC had the same drop rate. This is just you looking back erroneously.