It is indeed about grind. There is no lose condition. Just keep playing for long enough and you will eventually win. “Long enough” may be years, but it’ll happen.
(Unless you’re on hard mode, in which there is a lose condition that feels like the afterthought it is.)
I say this as someone who has hundreds of hours in Darkest Dungeon, played long enough, and eventually won.
In a way, Darkest Dungeon is not at all like a roguelike. Individual characters may die, but your game continues, and the Hamlet lives on, with an endless parade of fresh meat. This game doesn’t have permadeath. It has permalife.