At what time does the line get crossed where it becomes just bots interacting with other bots?

It happens already and doesn't really matter.

The floor traders of stock and commodities exchanges have almost entirely been replaced by algorithms that make the same decisions in milliseconds and with far less hypertension.

But without human desires every market trade would still be meaningless.

Who takes the place of traders on the floor?

Media outlets—distributing the human interactions parallel to the trades.