It's just soup. It's not like it scalded the OP or left them with horrible burns. They said themselves it was no big deal and didn't want to make a fuss. I don't necessarily think "whining" is the right word, but can you really be upset about not getting free stuff after you yourself insisted it was no biggie?
I don't think people are trying to force a thread backfire here. In my experience, comps are usually given out to calm down upset customers and keep them coming back if it feels like their experience was beyond redemption. OP sounds like they were a pretty chill customer and didn't make a big deal out of it, so it's weird to then be upset afterward that they didn't get anything out of it.
Meh, in my experience, waiters and managers usually go out of their way to compensate you somehow when something like this happens. They offer a free dessert or extra food on the side, or when the bill comes cross out the item that they messed up, or something. I don't feel like a customer should have to complain to get offered something as a gesture. At least at good restaurants that care about customer satisfaction.
And on another note, I'm also weary of complaining in restaurants. Makes me feel like I'm being an inconvenience or as if I've done something wrong. I know it's not the case, and if I do complain, I try 100% to be super nice about it, but I've read too many accounts of people that used to work as restaurants saying how annoyed they get at customers complaining. Oh you said you didn't want onions in your salad? And you couldn't just move it to the side and instead complained to me when you see I'm busy with several tables? well how about I put some special sauce in it...
Obviously not all waiters would think like that but I know people that do that worked in in restaurants, and yeah, I try my best in all circumstances to not complain to the people preparing my food behind a wall.
I really respect and tip extra to the food establishments that make mistakes and compensate you on their own without having to be complained to.