Despite the downtrodden title of this post, this is actually a life-affriming statement of positivity.
...with a bit of bitching thrown in.
So! Let's explore the idea behind the title's premise: all editions of 40K are broken. Yup. Let's break down each edition and figure out what made each edition so memorable.
Rogue Trader/1st - Never played, but from what I can gather from older grognards this was little better than a roleplaying game. You needed a game master to run it for you. Hardly a game that resembles what modern 40K is. Broken because basically anything went from a narrative standpoint. It wasn't balanced but it wasn't supposed to be; it was a way to have fun with models.
2nd - Where the game becomes a real game. Army books and lists become a thing, as do Wargear Cards. Herohammer at its height, powerful characters are all but invincible. Chaos is an unbeatable wave of death. Orks have Pulsa Rokkits.
3rd - The rulebook contains several "black codex" armies that are invalidated by later codexes. Assault is a mess, which hampers assault based armies. Combined with transport vehicles, the "Rhino Rush" rules the day.
3.5 - Updated assault rules make assault a very viable tactic. Blood Angels Death Company and summoned Daemons (especially the White Dwarf rules for Steeds of Slaanesh) make the game about whoever gets the charge. Tau had to be FAQ'd to say that Fire Warriors block line of sight to Suits or Tau would just die.
4th - The Rhino Rush dies a horrible death as Entanglement allows even Fearless units to become pinned! Gunlines are the mainstay. Guard shoot the sun at the enemy. Chaos tools out Daemon Princes with 150pts of wargear and lets them dominate everyone in the face. Vehicles become tombs.
5th - Entanglement is gone, but units can no longer assault out of vehicles. Firepoints turn transports into mobile gun platforms for infantry. New wound allocation rules allow certain units to redistribute wounds creatively, allowing the rise of the "Deathstar." True Line of Sight terrain rules are introduced because apparently having an imagination is awful. Due to this, forests became harder to model in 40K so the focus shifted to line of sight blocking terrain such as buildings. Only troops can hold objectives. "Kill Points" favored small elite armies.
6th - Random crap becomes common. Roll for psychic powers, warlord traits, charge ranges etc. These random elements make psykers less attractive due to the inclusion of Deny The Witch. Assault is now for losers as random charge ranges make even a close ranged charge a possible failure. Overwatch gives even more penalty to assault. Shooting is king and Tau are the best at shooting so good luck beating Tau. Flyers make a debut and we see necron air lists and chaos dragon lists that dominate.
...so even with all these things broken why is this a positive message? One word: FUN. 40K is supposed to be fun. It will never be perfectly balanced. There's always going to be a metagame and that meta will favor some armies. Part of the fun is making lists to deal with the meta.
This is why I'm optimistic for 7th. Even though it won't be balanced. Changing the edition shakes up the meta and makes certain ideas that were valuable useless, while making new ideas better.