Why do you think that INNO should do more than it already has to fix a "problem" that is caused by the players themselves? This debate has been going on for years. Some players choose to join "laid back", "low key", guilds that have low expectations of their members. If these players really want to compete in GBG all that they need to do is join higher ranked guilds.
Clearly they don't want the pressure of being in a "top" guild for one reason or another, which overrides their desire for GBG success. It could be the specific top guilds on their world - it's not like there's a lot of options on any given world. Or just that they want to show up when they have time, which is not necessarily enough to meet the expectations of a top guild (or at least they feel it's not); and even if they were in a top guild there might be nothing to do when they show up as top guilds are routinely subject to internal competition to get fights.
It's likely that inno would want to do more about the situation in GBG themselves, if they felt it could indeed be fixed. But shrinking the number of diamond guilds would likely aggravate things further as it's not like there's great parity at the top, and the happiness of the #4 guild is likely to some extent connected to not always seeing #1-3.
Their latest change actually went in the opposite direction where there's now more 1000 guilds and some extra minimally active guilds are in fact able to stay in 1000, for better or worse. The "super" rounds do have excellent matchmaking though in my experience where the lower groups are very evenly competitive giving those low diamond guilds a chance for a fun and fair round.
If anything needs to be fixed atm about GBG, I'd say it's the gameplay itself where it essentially boils to down to mindlessly hammering hotkeys. But at least it's the most-optional feature in the game right now such that if that doesn't appeal, skipping it really doesn't feel too bad
That's probably the best change to GBG even if it wasn't directly to it
