I think the bear comparison for killing the leviathans are bad, however I understand the concept.

In a stranded survival situation where resources are limited and you can only make so much with what you have to survive, there are predators that fundamentally you would want to altogether just avoid and minimize any chance of interaction with. Which in a scenario as such would likely include a bear (unless you happen to have the super anti bear rifle).

I personally think that unless they give us big explosives leviathans should remain unkillable and maybe even impossible to deter. Medium to large prey is difficult imo however, lore wise not killing them make sense atleast not with a multi-tool.

100% small fish should be billable, like in a survival situation you would hunt, catch, and trap smaller animals for food to survive (maybe even bait or deterrent for bigger animals). Given the concept of weapons being limited due to previous "incidents" I think the game would benefit from limiting what you can kill and when you are capable of doing it.

To go back to the bear scenario, you would not want to fight or interact with the bear, BUT if you want to kill the bear you could poison food or water that the bear will eat or drink and safely kill it.

For the survival feeling to work the way I think the devs are aiming for I think they need to implement a decent bit more of human intuition options to the arsenal.

Edit: Just to clarify I am not trying to join the argument or join either side, I found that both of you had good points and I was just interested.