I'm a patient person, so even if I'm not enjoying a game immediately, I'll usually finish just so I know I've had the full experience with the game. For example I just beat the The Last of Us, which I initially felt was outright terrible (I know I'm an outlier), but ended up being decent and worth my time. That being said, I don't really have enough gaming time to bother with anything other than the most acclaimed titles, so it's rare that I encounter a truly terrible game.
The only exception is where I get too busy to play a game and it's not very convenient to play it. I'm way more likely to finish a game on Switch than on PS4 because I know I'll find more opportunities to squeeze in some play time.
As for DLC, I almost never touch it, even if I loved the main game. As I said, I don't have a lot of gaming time, so even if I'm loving a game, I start to feel pressure to move on to the next title in my backlog. As a result I end up getting frustrated when games are really long, even when it's not due to bloat. I end up skipping side quests, mini games and even game mechanics that I wouldn't otherwise. So as far as I'm concerned, if it wasn't important enough to include as part of the main game, it's not a critical part of the experience.