Sacrosanct
Legend
I''ve seen this complaint, and I don't get it. That's like griping that an eldritch knight uses his weapons too much. They get, like, NO spells. What else are they supposed to do in a fight?
Well, I think your basic premise is flawed in that you're assuming a warlock is supposed to do as much DPR as any other class. I.e., maybe a warlock doesn't have to be good at combat in the first place (maybe because they do a bunch of really cool things out of combat). Secondly, maybe there are things to be "good" at combat other than DPR. Polymorphing objects to affect the battle. Or silent image. Or confusion. Or hold monster. And you still do have spell slots if you really needed them.
Speaking of things I don't get, I don't get this idea that unless you're doing X DPR per round, you can't do anything in combat. Creativity and role playing don't stop when combat starts.