You can do what you like except for what you can't. Naturally a clever enough person with the resources could design an effective plague and knock numbers down. I shouldn't be surprised to see that happen. Regarding my comment starting with "A", there are movements which support the elimination of humanity for being "unnatural". I submit that humans cannot be that because we are the product of changes in the environment.
Nothing lasts forever, however we have evolved to be THE technological intelligence on the planet. We have hands and the evolved ability to manipulate the environment. Consequently we naturally make weapons to compensate for our unimpressive physical abilities with encountering other animals. If you were to remove the natural adaptive ability then yes we would likely be extinct. But a human male in good condition armed with a well designed bladed pole arm could overcome a tiger. Considering our prehistoric neolithic ancestors hunted megafauna it follows that we have done well against creatures what would easily have killed us if we eliminated our natural abilities to make implements for whatever reason.
I submit that the problem isn't humans being superpredators but that wisdom is in short supply and that discernment has not kept pace with our ability to build. We constantly find ways to do a thing but fail to avoid those which are harmful.
You're right- I say the same thing to nationalists- nothing is meant to last forever. This includes nations. If I was given a choice between starting a nuclear war to try and save my country (but possibly destroy the planet) and not doing it and risk being conquered, I'd choose the entire planet vs the existence of one country, even if it was mine.
Not really referring to other animals in my point about extinction, but moreso the type of food we eat and the higher rates of environmental pollution as well (which simple organisms evolve far more quickly to become immune to dangerous chemicals and pesticides than humanity ever could.) Rates of cancer and heart disease have been increasing due to the consumption of processed food (there are statements about it put out by the ACS and AHA.)
I'd like to make our population lower but don't support immoral ways of doing so. I don't know how far lower birth rates will take us, but I think the rest will be made up from disease, drought, famine, wars, etc. The only way out I see from us becoming extinct one day is escaping this closed system via space colonization.