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?
Definitely. Autocracies always end up being poorly run. Any system that concentrates all authority in a single ruler is going to have some pretty bad outcomes. Even if the dictator really was the smartest guy in the country, instead of merely the most ruthless, even geniuses make bad decisions from time to time. Autocrats quickly find themselves surrounded by yes men. This is how you end up with boneheaded ideas like Mao’s backyard steel production or Stalin embracing Lysenkoism.