I think your questions are more complicated than you realize.

Are Autocracies more powerful than Democracies?

If you separate the form of government from the governing, yes autocracy is a superior form of government. A dictator can instantly marshal resources to face any threat, or completely shift an entire nation, if a direction becomes clearly wrong. The reason they don’t work, is because the leader is always human. Humans make shit leaders, almost always. So distribution of power across a large number of people mitigates the risks of putting it all in one.

Are all democracies are doomed to fail?

Yes. Obviously. Everything eventually fails. The Sun will fail and take the earth with it.

Is the future of humanity, autocracy? For the rest of humanity’s existence?

No. Obviously. Everything eventually fails. The Sun will fail and take the earth with it. I would hope humanity (or whatever species humanity evolves to) lives past that.

I just call it a “Pull Cart”.
Or “Cart” for short.