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?
No.
Autocracy moves faster at marshaling the resources it has, but is significantly worse at accumulating resources than what economists Daron Acemoglu, James Robinson, and Simon Johnson describes as inclusive political and economic institutions, which broadly allow members of the public to engage in political and economic activity. (Note that their work on these things won them the Economics Nobel last year.)
Distributed, decentralized power is important for maximizing the potential of a population.
Autocratic political systems are brittle. They’re also poor. They tend not to survive more than a decade or two before the strongman is deposed, one way or another, whether from internal coup or revolution, or simply external invasion of a weakened state. And a successor strongman might be weaker. All the while, the inclusive states continue to grow in their own power and influence.
So any short term gain in consolidating power into smaller groups is going to be up against time, and the fragility of the whole arrangement as the autocratic country falls behind its competition.