Somehow, I think 4e was too progressive for most of the fans.
I don't think it is a matter of being progressive. Progressive is not automatically equal to superior.
It's a matter of quality across the board (both crunch and fluff), not just quality in a few areas.
I'm an old grognard, but even I can see the advantages of many of the 4E changes. But, I also see many of the disadvantages whereas I think that your Progressive hardcore 4E players might not see those.
Good change is good. Meh or bad change, not so much. Everyone has a different opinion about what is good and what is not, but 4E didn't just change crunch. It changed some very significant (to some players) fluff as well.
I'll give a simple example of what I consider Meh (or even bad) fluff change in 4E that many people have probably never even heard of.
Dragonborn in 3E were creatures of any humanoid race that heard the call of Bahamut. They were originally Dwarves, or Human, or Halfling, or whatever. Bahamut changed them into Dragonborn.
Now, most people were not heavily invested in 3E Dragonborn. They were only around for about 2.5 years before 4E came out. But for a player who had a very intensive backstory for his Dragonborn PC, 4E Dragonborns could be a very annoying shock. How dare WotC just up and wipe out the entire Dragonborn concept and replace it with a new one such a player might ask?
Quite frankly, I cannot think of a good answer to this question. WotC just changed it with no real good reason as far as I can tell. WotC did even more with regard to Paladins and Wizards and many other classes and races that a lot of players DID have a heavy personal investment in.
So to the vast majority of 4E players, this small group of 3E Dragonborn players who might be bothered by such a drastic change (in their minds) is no big deal. Live with it. But, it's the trampling of the D&D concepts which as a DM and player for almost 35 years, I find to be most annoying. This is merely a simple example. Not only did 4E totally revamp a significant portion of the crunch (wiping out 30+ years of many core D&D ideas), but they revamped a significant portion of the fluff as well.
WotC just threw these players (and many others) under the bus, all in the name of progress. They didn't really care that they were wiping out many perfectly good PC concepts. Their "problems" such as the 5 minute workday, Codzilla, Polymorph, and golf bag of healing wands were more important to them than the D&D feel consistency of their fan base. They didn't solve the problems by staying within the fluff, they did it by throwing fluff (and crunch) away and creating new fluff. Sometimes to match the new game mechanics, but sometimes just because they felt like "being progressive".
That's a mistake and I think that's why 5E is heading back towards that D&D feel consistency.
I actually applaud WotC for trying to merge original fluff and crunch back into the game. They won't be able to satisfy everyone no matter what they do, but could you imagine if they were to create a 5E that was even MORE progressive than 4E, but further away from 1E through 3E? They would not only have lost a significant 1E, 2E, and 3E portion of their base, but they would push away a portion of their 4E base as well.
By merging a lot of the concepts of 1E through 4E, they might push away some of their 4E base. But at least they might also bring back some of their 1E through 3E base. No matter what they do, they would have lost some of their 4E base. At least this way, they can at least try to make up for that. Combining conservative with progressive might be the best decision that they can make for a 5E.