The problem is "cleave" has never been a good niche because it's used so rarely. 1 target with intermittent add spawns isn't true "cleave", any time you can't use Havoc on CD wastes the ability's potential. Any time targets are spread out makes FnB completely useless. And any target beyond 2 is no longer "cleave" but no is "multi target", i.e. Affliction's playground. Just check Coven rankings to see what happens on 3 targets, a cool and casual 94 Affliction Warlocks in the Top 100 on Mythic. But heck, there's 70 of them on Hounds, too, and only 10 Destros. (And before someone cries HATER, I myself am in fact a very happy Affliction player and just because I personally would be fully content if Destro disappeared into a chaotic rift never to be seen again ever doesn't mean I don't empathize and understand it has problems)
Anyway, this whole notion of "cleave specialist" is just terrible design because it runs into too many limitations. It happens too rarely. Most fights are either predominantly single target (the same or with priority) or are multi target to some degree (intermittent or longer duration). True, sustained 2-target cleave is rare. Having the entire spec designed around that is, consequently, a fairly stupid thing to do if it comes at the cost of ST performance (since you can't exactly let Destro straight double up on 2 targets) or target limitations (with Havoc CD and FnB range).
MoP Destro worked because it wasn't put into that niche so hard, and because there was a lot more flexibility. You could toggle FnB, Havoc had charges, you had a greater selection of heavy hitters, one of which was perfect for intermittent add fights (Shadowburn). That still didn't fix the multi-target scaling of other specs but it sure meant much wider application for the spec. Remember Throne of Thunder? Destro had its place there despite the silliness that was UVLS-Demo, which was just absurdly good.
Also: burst! Why can't Warlock have more of that? Does it HAVE to be all DoTs and drains all day every day? Demo has terrible target switching right now, Affliction has ramp-up (though admittedly not even that much...), and though Destro is undoubtedly the best at bursting it isn't exactly spectacular at it, overall. Could it really be that all the problems come from them taking away Shadowburn?
I think it's fine if there's tendencies for classes, but this whole "specialist" notion has got to go. Yes it's more homogenization, but being great at something that just so rarely becomes relevant at the expense of everything else... to the degree where even that specialty ends up failing as a result... is just not great class design. We don't NEED the "2-target cleave specialist". That's just not a useful niche. Find something else.