I think it's a stupid "lesson" to feel that hard control was bad, because it set them up to later make crowd control completely without any use, because they kind of had the attitude of a scrub mentality when many rpgs had hard control in them. Just you needed to provide means to counter crowd control that was modestly available. CO initially had solid cc but CC got nerfed so much in the hands of the player, while also buffed in the hands of mobs when players ignored cc resistance.
Even at the beginning CO's control was far less than that available in CoH. And I wouldn't go so far as to say it was a "stupid" lesson. CoH genuinely had brain-meltingly high levels of control which in any other game would have made it stupidly impossible to balance very well.
It only worked in CoH because we were allowed to become so powerful that pure offense was competitive with hard control, except in situations where hard targets were not easy to kill. Thus: purple triangles. But even so, consider how controllers complained their primary was significantly attenuated in Incarnate content. And conversely consider the flip side case of putting the STF AVs to sleep. It was never a stable situation in CoH.
What Cryptic failed to learn, what even Paragon didn't always appreciate, and what few game designers seem to acknowledge, is that "game balance" is a context-sensitive term. There's no such thing as "numerically balanced" in the literal sense, because numbers offer no guidance on balance. The definition of game balance I developed during my time with CoH was:
Balance is about making game play options have the valuations they need to have in order for the gameplay to function as its intended to function. Notice three words I don't use in my definition: "numbers," "equations," and in particular "equals." Game balance is not when something is equal to something. Its when the relationship between X and Y are what they need to be for the game to work properly. Not necessarily equal.
Hard control in CoH wasn't as bad as it could be because it existed in a world of high-order AoE, of aggro control, of super-high defense. It existed in a world where high defense and aggro control could neutralize a similar amount of damage, and where high-order AoE could defeat as many foes and hard control could nullify. It was broken, but it existed in a system of lots of other broken things that happened to (very roughly) balance out.
The lesson Cryptic should have learned, the subtle lesson that escaped most players as well, was that gameplay balance is about presenting equal opportunity options to the player, and the components of those choices must be engineered to offer those choices. In CoH, you should not be punished or overly rewarded for picking the "right" or "wrong" archetype. There should be no right or wrong archetype. And that means the things to give those archetypes must give the player a unique but roughly even ability to succeed. If you're going to give out super-strong defense and aggro control to one archetype, you have to give out very good crowd control to another, and very high damage to the other. It wasn't always like that: it was highly unbalanced at launch. CoH was refined to become that over time, at least to a first order approximation, because Cryptic/Paragon had no choice: they couldn't start from scratch. But the choice they were forced into was actually the best one for players.
I tried to explain this philosophy during CO early beta. It did not go well.