Humans aren't giants, they stood on the shoulder of other humans. So for AI to be equivalent they should stand on the shoulders of other AI models.

building for thousands of years with a population size in the range between millions and billions at any given time.

Right, and when we have AI that can do the same with millions/billions of computers then we can replace humans.

But as long as AI cannot do that they cannot replace humans, and we are very far from that. Currently AI cannot even replace individual humans in most white collar jobs, and replacing entire team is way harder than replacing an individual, and then even harder is replacing workers in an entire field meaning the AI has to make research and advances on its own etc.

So like, we are still very far from AI completely being able to replace human thinking and thus be called AGI.

Or in other words, AI has to replace those giants to be able to replace humanity, since those giants are humans.