I actually had a warlock in my group that did amazing damage just yesterday. I really don’t see a reason to not invite them. Especially at the level at which you’re trying to progress.
Keep in mind that there might be other reasons for being declined aswell. It’s easy to blame your class, but honestly I’m having the same experience on my rogue even though it’s considered one of the more meta classes. There is just a massive oversaturation of Dps players. While your gear is good enough there will probably be people with better gear queueing for the same dungeon.
10+ is a point where a lot of groups start to struggle, so people will be more inclined to just wait for overgeared Dps. In my experience you don’t have to wait too long to have vastly overgeared players apply for your group.
Your best bet would probably to create your own groups or join a community dedicated to mythic+. While these communities aren’t a solution to the overpopulation of Dps, they atleast won’t decline you for your class choice and in general offer a more friendly atmosphere.
Communities that come to mind are:
Alliance:
Horde: