It’ll start fading at endgame. I would say at the start of BFA but you won’t really be seeing the insane power loss we had after losing artifact weapons so you should be fine there.
The thing is, Destro is good at raids right now, but ONLY because Destro gets a lot of power from Visions of Perfection. Until you get rank 3 of that, as well as a solid set of azerite powers, you’ll find you only really do good damage during your Infernal, which is a 3 minute cooldown. Between Infernals, you’ll comparitively hit like a wet noodle. Visions of Perfection fixes this by allowing fairly consistent yet highly RNG minifernal procs. Come Shadowlands we’ll lose all that and suffer heavily if nothing is done.
The real problem, however, is the rest of the endgame features. The rest of endgame content are basically time trial content, M+ and Visions. None of the Warlock specs are actually any good at either of these, due to lack of mobility, not great or clunky AoE, and subpar utility (yes we have good utility, just not for M+ or visions). Demo is the best for M+ and even they aren’t desirable.
It’s not that Warlocks CAN’T do it, we absolutely can do high keys and solo 5 masks, it’s that it takes way more effort and is way more difficult to do compared to most other classes. Even having a pocket tank in Visions is not much of an advantage as VW and Felguard will die quickly there (not to mention VW can’t hold threat off of a Destro lock at all). For M+ you’ll be ok if you have your own group of friends to run with, but pugs will be very difficult to get groups with, and once you’re in it’s pretty easy to see why Warlocks generally aren’t wanted as you find your ramp up damage is ill suited to the fast paced nature of M+, and you won’t be able to do anything while moving, and you’ll need to move a lot.
Don’t let this discourage you from having fun with leveling, mind you. It’s ok to have fun, and we’re hoping beyond hope that SOMETHING will be done to fix Warlocks in Shadowlands, just be wary that we don’t mesh with endgame at all.
The problem is, at its core, all 3 warlock specs are actually very well designed and fun, BUT none of them actually mesh with the type of game WoW has become. Because of this, new time Warlocks find the class very enjoyable, very strong even. But the more you push with Warlock, the further in you get, the more glaringly obvious Warlocks flaws become. More so if you’re familiar with what other better classes can do, and even more so if you are a long time Warlock and know everything we’ve lost over the last few expansions. We used to be a solid class. Now we are functionally broken and rely entirely upon the expansions borrowed power system to try to fix our broken class. And let me tell you, when Shadowlands rolls around, a LOT of warlocks are going to feel the loss of our azerite powers, and it is going to feel AWFUL. Every class will to a degree but I daresay Warlock will be hit the hardest.
And our biggest fear? Nothing will happen as none of the devs main warlock, so in the process of testing, they’ll see what you do: a fun powerful class that feels good to play in leveling and low end content. They won’t see the problems only experienced warlocks do, and as such we won’t be fixed. Especially since Blizz has a long and consistent history of ignoring our feedback. Like, Destro warlocks had to have their entire resource system completely overhauled mid-Legion because they needlessly homogenized soul shards across all 3 specs and Destro was just not designed for that system. We told them it was bad and needed to be reverted all through the alpha and beta but nothing was done until iirc just before Nighthold was released. It took an entire raid for Blizz to notice what we had been telling them from the alpha.
Eh, sorry, I think I turned this into a bit of a rant. Just, go ahead and have fun with it, it’s the end of an expansion, after all, and is the perfect time to branch out and experiment with alts, just… be wary about the endgame.