If this "one class can completely overshadow another class and be better in nearly every way" how is it in our Curse of Strahd West Marches campaign that started in January we've had 42 characters created (so far) by 15 players and we have not a single Twilight Cleric? Are the players just plain dumb and/or not playing optimally? Or are they all really smart and in silent solidarity on opting out of picking a Twilight Cleric b/c they see it would ruin the game? Yes, one table does not prove anything, but it's not like we don't have any power gamers at our table. So, there's that, FWIW.
I dunno. How many encounters is the average DM running per rest? If a 1st to 5th level Twilight Cleric gets to shine (yep) for one combat per rest, does it really break anything? I guess if you are only running one combat per rest. The game designers, IMO, cannot be blamed for a lack of strategy on the DM's part (aside: in case anyone hasn't seen this helpful resource:
The Monsters Know What They're Doing)
Sure, I'd give them their early "easy button" wins but, after a few encounters, word would get around in the game world about said Twilight Cleric. And then I'd have the baddies trigger the channel divinity then run away for a few minutes until they weren't so shiny. Etc.
And, in conclusion, I reserve the right to change my opinion once a player inevitably brings a Twilight Cleric to our table and wreaks havoc upon our fun in a way I hadn't considered yet...