Hi 
As a long-time Warlock player, I basically agree with you.
I’m mainly talking about Destruction Warlock. Right now, Destro’s performance in PvP isn’t bad however, performance ≠ fun. I think I speak for many Warlock mains when I say that playing Destro in arena is often not fun, mostly due to its limited mobility and the fact that its burst is heavily tied to Chaos Bolt.
While abilities like Shadowburn, Wither, Conflagrate, and Infernal do generate decent overall damage, the problem is that this damage isn’t “dangerous” damage. You rarely force enemies to use defensive cooldowns with these spells or create meaningful pressure on the enemy healer. If you want to do that, it’s all about Chaos Bolt.
The issue with Chaos Bolt is that without a stack of Backdraft (which can be purged), it’s a hard-cast spell that costs resources. Even when you manage to successfully cast it, Chaos Bolt hits for only around 4M damage on average (at 722 ilvl).
Another big problem is Destro’s limited mobility, which requires a setup before you can even move. Demonic Gateway, without the PvP talent, is short-range and has a ridiculous 90-second cooldown. Demonic Circle is decent, especially with the Impish Instinct PvP talent, but it’s still not enough.
Possible solutions for Destro would be to bring back the Conflagrate root. Spamming Curse of Exhaustion on classes with extremely powerful gap closers feels pointless. Another good solution would be to bring back the beloved Kil’jaeden’s Cunning.
That said, I think sooner or later Warlocks will follow the same design direction as Elemental Shaman or Balance Druid, where your class can pump out tens of millions of damage without ever really casting.
Unfortunately, I think the “tanky caster” fantasy is outdated in modern WoW.