You’re talking as if (theoretically) LoG, KSE and Trivium would headline the same year. They wouldn’t. Chuck them in alongside other headliners in the future and it could work.
If you break it down into (loose) groups, the headliner pool can look pretty healthy going forward:
Old reliables & returning acts: Metallica, Iron Maiden, Linkin Park, Slipknot, System Of A Down, Avenged Sevenfold, Bring Me The Horizon, Biffy Clyro, The Prodigy
Bands Green Day have opened the door to: Blink-182, Paramore, Foo Fighters, Nickelback, Nine Inch Nails
Legacy bump ups: KoRn, Deftones, Limp Bizkit, Trivium, Bullet For My Valentine, Lamb of God, Killswitch Engage, Machine Head
New bump ups: Sleep Token, Ghost, Parkway Drive, Bad Omens, Spiritbox
They're very crude groupings but, in this hypothetical scenario, you can see how mixing acts from all of these groups could create fairly balanced headliner trios going forward into the next decade.
Again, I’m not saying I 100% believe all of these acts will headline (especially LoG, Trivium etc.) but I am saying that, based off of the elevation of acts like KoRn and likely Bizkit and Deftones, I wouldn’t be too stunned if they eventually did.
To keep this threat going and start some speculations about stages and slots:
I put together announcement posts reactions, combined number of likes from Fb & Insta.
Of course, this is not a completely representative sample and is influenced by a number of factors, but it still says something
Massive Attack: 9960
KOL: 6050
Kiwanuka: 2.7K
Fontaines DC: 2620
St. Vincent; 1820
Yard act: 590
MA are perfect INmusic booking. Fontaines are missing bigger fanbase in region to be solid headliners, mayce co-head with another solid size name? Kiwanuka seems popular, he is also listed above St. Vincent on festival web, he might be on main stage this time? Just like St.Vincent was in 2018, any chance she will "downgrade" to word stage or is 20.30-22h main stage slot nearly full?