Not to sound too "elitist" "smug" "patronising"
... but apart from to very new players when have 5 mans ever been hard? I started in TBC and yeah, there was a time period when I was 70 and found 5 mans hard; but then again I didn't have spells like "cleanse" key bound at that point in my game time.
TBC heroics were never hard; players were just inexperienced. Cataclysm heroics WERE NEVER HARD. They weren't PuG friendly; but then not a lot of content bar LFR *is* PuG friendly; and I'm sure we all have mixed opinions as to how successful or beneficial that has been. But to people who had any PvE group experience, they were very much the same as in previous expansions.
Remember when Halls of Lightning heroic was "hard" and you could look up strategy guides on how to beat Loken? Yeah, so do I; does that mean Wrath heroics were "hard"?
Introducing or "re" introdcuing hard heroics will just be a frustrating time-sink for content that very few people are genuinely interested in. For most people 5 mans are a bridge into end-game content, not end-game content itself. Sure there are people like the OP for whom 5 is the perfect group size; but like the selfish ass I am; I'd rather they added an extra raid boss than an extra 5 man, or spending time making the 5 mans balanced and challenging. It would certainly garner more of my in-game attention killing an extra boss than running a 5 man two or three times.