An autocratic country could easily spread propaganda in the democratic country, because of “free speech” rules that most democratic countries have, but a democratic country cannot easily spread its propaganda in the autocratic country.
An autocratic country can buy an election in the democratic country, but the democratic country cannot easily coup an autocratic country.
Are all democracies are doomed to fail?
Is the future of humanity, autocracy? For the rest of humanity’s existence?
This is an extremely nuanced topic with thousands of mitigating factors affecting it in the real world and subjective definitions about what “power” means. “Doomed to fail?” What sort of a loaded question is this? Isn’t every civilization “doomed” to fail given the ultimately finite nature of life? If a democracy does fall to an autocratic uprising or another autocratic civilization, was it “doomed” to be so?
This question is just way way way way way too broad and to expect any sort of narrow or succinct answer. I don’t believe we can really say any form of government is more “powerful” than others or that any are “doomed” to fail (or we can be reductive and say they all are).