… what

No, you’re confusing finite AGI for an AGI that can only compete with a single person at a time.

In a world of AGI better-than-humans at both A and B, it’s a whole lot cheaper to replicate AGI that’s better at A and B and do both A and B than it is to employ human beings who do B badly.

Again, you're just assuming you can infinitely replicate AGI, and there's no reason to think that's the case. It's only always cheaper to replicate AGI if there is no resource constraint on doing so.

AGI that is competent enough to perform any task better than a human, can perform tasks of controlling robots and mining equipment and construction equipment and factories and compute foundaries, to create more robots and mining equipments and etc. etc.

Look up into the sky tonight. That glowing white-ish disk is made of the resource constraints, and is a tiny fraction of the resources available.

It may be a finite quantity of resources, but with regards to the economic impact it's enough that it may as well be infinite compared to what humans can do… and what we can experience as a consumer. I mean, sure, a McKendree cylinder is bigger than an O'Neill cylinder, but the scale of what's being suggested by AGI is literally everyone getting their own personal fleet of 60 or so O'Neill cylinders — enough to spend a lifetime exploring, to be a playground for anything any of us want. Demand gets saturated.

And even that's not enough.

From the other side of the equation, we humans have a basic energy cost to staying alive.

There are already domains where AI can do things for energy costs so low that it isn't "economical" to pay for the calories needed to keep a human brain conscious long enough to notice that it has seen the result, much less to do that task.

Should that become true across all domains, there's no comparative advantage to keeping us alive. Hide the farm where your food grew behind PV, disassemble the sand and clay in the soil into constituent atoms and use the silicon to make more compute or more power.