Some of the hardest single-group content in MMORPG history was the on-launch Cataclysm heroics and people were farming them after a few days. You simply cannot give that sort of content the difficulty curve, mechanical complexity and length of relevancy to really qualify as progression.
No.
Cata heroics were a fucking joke for anybody with two arms, don't fool yourself. Vanilla WoW was a lot harder (in relevant gear of course) because of a simple thing: the tools given to the player. Warriors (the only real tanks back then) didn't have any single ability to control threat AoE and had to juggle sunder armor, demo shout and battle shout to get a bit of AE threat, CC was useful in the heat of battle because few things were AE'ed down to pieces on pull and dps classes couldn't self heal nor take more than a couple hits before eating dust, save for defensive skills being available and correctly timed.
Yes, you could use a bear druid to swipe three (3!!!!) targets at once, but that was it and it took a whole lot more damage than a warrior of equal gear (assuming you managed to find decent agi gear for tanking).
Let's move on a bit: Rift had challenging dungeons at the beginning, so did LotRo, even if most of the challenge came from stupidly high healing threat and shitty healing spells in this case. EQ2 had several very challenging dungeons in its history, I played for example through Nizara and Castle Mistmoore where you needed flawless execution to compensate for lack of gear (CM was a bitch without raid gear to be honest).
What makes MMO content "challenging" is the number of hoops you have to jump through, while doing your job of tank/heal/dps. These hoops can be, not necessarily in this order:
- resource management (mana/energy/whatever)
- number of adds
- special effects every defined unit of time (floor burning)
- mob(s) AoE
- debuff on tank
- debuff on healer
- stupidly high damage on tank
- stupidly high damage on everybody
- must-kite adds
- must cc adds (when CC wasn't just limited to a single mob per player it was even harder)
- "bring this add here to be killed so it triggers event X or removes buff Y" mechanic
- coordinating multiple things listed above
- more and more according to the designer fantasy
Of course if you make a 4 people party (SWtoR) it's going to be difficult to ask for many of these things to happen at once, with five (WoW) it's a bit better and with six (EQ-EQ2) in my opinion it'd be pretty much perfect.
EQ was a pretty easy game in its day to day group-hunting sessions, except when trained hard or when a respawn/overpull/glitch happened and suddenly you had anywhere from 6 to 8 mobs in the middle of your ranks. EQ allowed for good players to shine and save the day, through a combo of CC, tank responsiveness, cleric using root/barriers to survive long enough and stabilize the situation, enchanter/shaman slowing and again enchanter CC'ing, necro/ranger kiting stuff, etc. A shitty pull in EQ used to be quite entertaining (admittely you had time to think, because every action took quite a while compared to modern games where you're more likely to be GCD locked more than anything else).
What I'm saying is that nobody made a successful end game with 1 group content because nobody ever tried. I'm not saying they would have to, after all MMO are also about socialization (something a lot of devs forgot on the road I guess) and large raids help forming large guilds that promote this socialization.
LFR-like activities are honestly junk and should be deleted because they are not fun, in fact they almost always generate opposite feelings, frustration and anger, but people suck hard dick for shiny purples that are good for nothing but to face the next rank of frustration and anger.
Flex raiding (I never used it as I don't play WoW anymore) is what guilds asked from 2005 onwards, and should be used and even improved. It promotes coordination and effort at least, instead of cruising semi-afk to loot. So if you can Flex above 10, you can likely flex a bit below 10 as well. Groups of 8, 7 and 6 should be challenged by the same stuff that challenges 10 people.