On the other hand, the opposite of salvation, damnation, is easy to conceive of.
I would argue that because salvation is nebulous and damnation visceral, that the teaching of salvation as a concept does more actual teaching of damnation.
Damnation thinking does not get people to behave themselves. It creates judgment, one-upmanship, and suffering among people who are told that their human thoughts and actions are damnable.
Damnation-based religions are clearly not generators of human morality. History shows they are are continuous generators of human suffering.
The question becomes, is there a way that a salvation-based religion can be stabilized so that it won't mutate into a damnation-based religion?
Given the harm done to human lives would it not be wiser to simply not employ these concepts.
There are ways of expressing the need for mindfulness, care, and compassion that don't so easily turn toxic.
The above comments suggest (to me) that damnation/salvation is a teaching tool for cultivating morality in humans, but I think many religions wouldn't see it that way. They believe they are conveying some objective truth about the world, so damnation isn't a concept to be employed, but an aspect of reality. Ignoring it would be as harmful as ignoring climate change; they might say sure, we should all take care of the Earth because it's the right thing to do, but let's not ignore the potential terrible consequences that are on the table, too. Before people can be convinced that we've outgrown the concept of damnation, they have to be convinced that it's merely a concept in the first place.
To answer your last question from the OP, I think there will always be some segment of any salvation-based religion that approaches it from a damnation-based perspective, even if they don't realize it, and even if there are "stabilizers" in place. The only thing to be done is for people with a proper grasp on the theology to weed out damnation-based perspectives, and of course this raises the questions of what "weeding out" means and what a "proper grasp on the theology" means.
Also, I think it makes complete sense that salvation/paradise is impossible to imagine because it's not something we've experienced, a bit like trying to imagine non-existence. I can intellectually get my head around the idea of such a concept, and even be convinced it might exist, but I can't really imagine what it's like to experience.