The old game plan was tarpit our opponent in the center with lightning quick assaults from incubi, wyches and courts. We contested every objective and had units left over to score secondaries easily every single round. By the time our first assault is batted aside, it's turn 4 and there's no time to catch up in scoring.
The way the meta started to evolve, was these buffed up hard as nails, killy as hell units and models. We're talking GK/Nids, etc. Even with their ability to punch up, our units weren't taking them down in 1 shot and in response were getting mopped up the following turn. Add to that the shift towards kill primaries AND the points nerf, and the kabal/cult is starting to lose its appeal.
Where kabal/cult had a really strong speed/killiness point efficiency, coven currently has a terrifying tankiness/killiness point efficiency, and they aren't even that slow on these 9th edition board sizes. You can see why a few coven units, at least, will make it into any winning tournament list even after they're nerfed.
To my mind, a good list with minimal coven will still include grotesques, master haemies, and a healthy number of wracks/haemoxytes. This forms a core to contest the center that no one does better.
Aside from that, we need to be able to win board control and kill the nasty unkillable models reliably. My thinking on that is the list has to threaten to kill a dreadknight in both the shooting and assault phase. Coven spam usually manages this with 6-9 pain engines, but ive found a mix of ravagers, test of skill hellions, succubi, reavers, grots or incubi and drazhar manage the same kind of threat, if not the tankiness.
My picks for anti-tank duty: ravagers, the often overlooked void raven, KotBH raider lances, assault as mentioned above.
Point scoring/utility: outflanking hellions, mandrakes, 5-9 backfield reavers that zoom up when needed and, in last place, scourges.
Honestly, there's a lot of great ways to play drukhari. It says a lot about how OP the book is that we judge sub-optimal to be anything under a 70% win rate tournament list.