First, let me direction your attention here:
https://www.frontlinegaming.org/2016/09/29/why-do-deathstars-work/
It's a nice little article that, very factually, explains that the main reason Deathstars work is that, when you have something very strong *and* which uses a lot of your army's points cost, then buffing that thing further has a compounding effect. Mathematically, let's say you can increase how much money you have in two piles by 10%, and are able to do this twice, and for arguments sake we'll say you have $100. If you split this $100 into two piles of $50 each and increase each pile by 10%, you'll end up with $110 ($50*1.1 + $50*1.1 = $110). However, if you apply BOTH 10% increases to a single pile, you'll end up with $110.50 ($50*1.1*1.1 + $50 = $110.50). That means you gain extra value by focusing both buffs instead of spreading them out.
That is the crux of why Deathstars WORK. They're accumulating extra value by buffing not just the original unit, but ALSO buffing each previous buff. Giving Invisibility to a unit of Guardsmen isn't what makes Invisibility broken. Giving Invisibility to a unit that has 2+ armour saves, 2+ Invulnerable saves, which can already reroll those saves is broken.
Now, what would fix this? It would need to be some bottom-up design decisions at this point. First, much less buffing, or a way to heavily restrict these buffs. Magic The Gathering has a great self-balancing feature called the Colour Pie; that is, as you start trying to make stronger and stronger decks, you have to start branching out into cards in other colours, and that makes it harder to cast any of the cards in your deck, and makes it more and more likely that you start getting clunky draws - so even though you have the best cards, it won't matter if you can't cast them. Back when Factions were almost impossible to use together, deathstars weren't as much an issue because you couldn't easily combine things from outside your core faction - so if you had a weakness, unless your faction had an in-codex method of dealing with those weaknesses, you had to learn to play around them. There was more limitations as to what can buff what.
Unfortunately, because ease of mixing armies leads to more sales (and hey, that's not a bad thing!
GW needs to have sales to keep around), and because this has been around long enough, there are newer things in-codex that are so strong that they can form deathstars even without interference.
As such, there's only two possible directions:
#1 - Scrap & Restart. This is what they did in 3rd edition. They restarted from baseline simple, with all major armies having their own 4-6 pages within the rulebooks. And you know what? It was good! It focused the game like you wouldn't believe, and everything since has only been bloating on top of it like a boil on a Nurgling's bum.
#2 - Try to make everything more powerful so there's all at nearly the same playing field. It's hard to tell if
GW is doing this on purpose, as some armies are getting real buffs and can play at the top tables on their own (Codex Necrons, Eldar, Space Marines, Traitor's Hate, Genestealer Cult), or is just accidentally doing this, as many other armies see no upgrade, or sometime even downgrades (Codex Orks, Astra Militarum, Dark Eldar)!
Personally? I prefer option #1, because I do think we've hit a wall. Some people really don't like this though, because they like the game in its current incarnation. They like deathstars and games full of super-heavies and summoning spam, etc. They're not wrong for liking it, but I don't believe that the way they play the game is the way that's going to encourage the game to grow and be one of the best games out there. I believe there will always be room for the type of game they want to play (Apocalypse supplements and whatnot), but I truly believe that the game as it is right now is hazardous to its own growth.
