There is a huge difference in a class being viable for a dip an the class being viable for higher levels.
Warlock is a very good dip class, the problem with them is their scaling for higher levels.
I know that the standard Warlock dip is a 2-3 levels for Invocations and/or Pact Boon, but I've played warlocks into level 10/11 range without them feeling weak or underpowered, it's all about maximising the utility of your invocations and spell choices.
Once you get to 12+ and there isn't much else there beyond the Mystic Arcanum, but honestly I've never had a game that started at 1 get much beyond the Tier 3 range anyway (and dipping 2-5 levels into Rogue or Fighter for Cunning Action or Action Surge is almost always worthwhile).
I'll completely agree that mechanically speaking the class is pretty front-loaded and that you get most of what you want from it in the first 10-11 levels or so, but I don't really consider it one of the weaker classes in the game (particularly if Hexblade is being permitted), though I suppose a lot of the class's utility is dependent upon how frequently your group can/do short rest (at the time in question I was playing with a Monk and a Fighter as two of the other three party members, meaning that short rests were fairly regular throughout the gaming day).