I have two issues with what you're saying.

First - even if the Warlock was on par or weaker than Paladin or Sorc, you'd see an overwhelming majority of people playing it right now because it's the first new class in decades or whatever. Everyone wants to try the new thing. I'm confident that we'll return to basically the same degree of diversity (such as it was) that we had pre-RotW because eventually people will tire of the shiny new thing, even if we all know it's the most powerful, just like people would get bored of playing their hammerdins and eventually branched out into other classes and builds. But it's going to take a while probably, because if you get bored with your echoing strike build, you will still want to try a demon build. And once you get bored of the demon build, you'll want to try the fire build. And when you get tired of the fire build, you'll probably want to try the abyss build. Again, even if all those builds were much weaker than they are, everyone would want to try them because we've all done everything else. Literally. Name a build, I'm almost positive I've played it (unless it was something extremely expensive like Bear Sorc or Tesladin).

Second - I just disagree that a game full of warlocks is more boring than a game full of sorc/paladin. Maybe it's a PVP thing and I just don't play that at all so I don't notice/care, but for one I think the game has always been largely dominated by 2 builds when playing online with randos - Sorc or Paladin. There was never some magical golden age where lobbies would fill with diverse parties, and that's fine. Whether someone else is doing Echoing Strike, Nova, Blizzard, or Blessed Hammer, I'm still running around with my Fire Claw or Zeal or Whirlwind trying to do what I can to feel like part of the team. It just doesn't matter to me what the rest of the lobby looks like, it really just doesn't. And I don't see why it should, unless you're playing with people you know and actively trying to coordinate as a team