Second, Wrath Heroic Dungeons were easy. They were varied and easy and they worked perfectly for LFD. The problems arised when they introduced harder (true) heroics in Cata. They shouldn't have done that, true Heroics should be left out of LFD with Normals being the easy farmable mode for randoms.
Third, you did not need gear from Heroics to get into Karazhan. Heroics were standalone content.
Fourth, Scenarios aren't even close to the experience of Dungeons and they weren't designed to be. Dungeons are essentially mini-raids, they require a group of five with some coordination and have actual bosses. Scenarios are glorified group quests with snippets of lore that aren't designed to offer any challenge or need for cooperation, to leave group composition completely flexible. And, yes, you are arguing that scenarios are the same as dungeons as far as casual endgame content is concerned. You said so yourself : "and blizzard also tried making dungeons content Via scenarios".