Well, I am here to tell you "So was 7th".
"But 7th edition failed us" you may say. "In the end, it was a terribly bloated mess!"
A-HA! There you have it. The bloat! But we will get to that. My argument is there was nothing really wrong with 7th Edition as a set of wargaming rules. When they first arrived on scene it was to great praise and adulation.
To me there was nothing fundamentally wrong with the core rules of the same. And for the first half of 7th, right up until Codex Necrons IMO, there was really nothing wrong.
Yet today it is a miserable failure in many's eyes. Here is what made it so
1- Formations- These really were the beginning of the end. They must have been a move by GW marketing. That is really my only reasonable explanation for as to why such an imbalancing factor was added to the game. First of all they were largely inconsistent in terms of power and playability. Some formations cost over 2000 points and did virtually nothing where as other gained 10 USRs for units totaling up to less than 400. There was just no rhyme or reason to them. Secondly, they began to more and more often provided exceptions to the core rules. Exceptions such as
-Assaulting out of Deep Strike
-Not paying points for upgrades
-Shooting and Running
-Not rolling for reserves
-Essentially not having to take saves (rerollable 2+/3++ and 2+ FNP rerollable)
-Tons and tons of different rerolls
They really just were a step too far ultimately. Beyond that I felt they started robbing the creativity out of list building, but that's just me.
And lastly in regards to them, there were SOOO many bad ones that you could tell were just made to move models. Did anyone really want a bunch of possessed? I don't think so. This more than anything showed GW's marketing department had a hand in the matter.
2- Decurions- What was more game breaking than formations? Well formations of formations of course. All the same issues as above except expanded across an entire army. Who ever thought hundreds of free points in Rhinos or Upgrades was a good idea? GW marketing that's who. Because anyone actually familiar with games design would tell you that an asymmetrical imbalance is bad for a game.
3- Faction Imbalance. This is largely due to too rapid a release cycle for some factions and not rapid enough for others. Basically a handful of armies got A LOT of attention and GREAT rules and formations while others just languished in ignominy, getting few formations or updates (Grey Knights), and those they did were poorly designed (Orks) or amazingly over costed (Blood Angels) or a combination thereof. On top of that, some armies were just downright BETTER. Eldar, GSC, etc. vs things like DE, Harlies, BA and Orks. It created a system of haves and have nots, and left a lot of people feeling they were loosing just because the tools they had were subpar. Like bringing a knife to a gunfight. Or Orks to a game versus Eldar. Take your pick.
4- Independent Characters, Allies and USRs- If anything was "wrong" with the core rules, it lay in the combination of these 3 things. Sure the Allies matrix was better than in 6th edition, but really it still needed more work. There was no reward for playing mono-faction and Battle Brothers was still too powerful (even at the end after the Chapter Tactics FAQ fix). They were too easily abused too many USRs conferring from random ICs joining squads they were never intended to join (Space Wolves and Dark Angels!?!?!?!). 8th is flirting with this with only faction keywords preventing this right now, But the moment someone gets something that affects all
5- A lack of FAQ or Errata for SOOOOO long. The most tragic and avoidable reason of all. How many little obvious things could have been fixed early on that weren't? We went what, 2+ years without an FAQ? How much resentment and bitterness in players could have been prevented with timely fixes of issues? How much easier for TOs could their lives have been? I am just glad GW has turned a leaf on this and hopefully they really do stay on top of it for 8th. Whether you like their ruling or not, it is healthier for the game that way.
6- No consistent design philosophy. 7th edition changed flavours with the seasons it seemed and nothing was consistent in terms of power level between releases. Decurions is the best example of this. First nothing had them then BAM, Necrons came out with them. Then a little down the road BAM they got turned up to 11 with tons of free options and even better formations and rules. Then BAM they realized they were TOO powerful and turned them back down to useless (looking at you Angel's Blade). And that is just one example. The design team was just way to reactionary and receptive to the will of the playerbase (and likely marketing yet again). They needed to pick a style and stick with it at least through a complete release cycle before resetting. Unfortunately that ship had long sailed by the end of 7th and the only way to fix this was with a reset across the board.
So far 8th doesn't have these issues, and I HOPE it never gains them. GW just really has to stay vigilant on keeping a steady even release for ALL factions that are balanced, maintain a well updated FAQ and ensure that they stay on top of their Keywords. But bloat will inevitably happen to some degree. We can only wait, play and see, all the while hoping GW learned a lesson from the demise of 7th AND doesn't allow GW marketing to butt into rules design too much anymore.