If 1 person can use an excavator/AI, then a 100 person crew can use 100 excavators/AIs. Capabilities have increased for everyone, not just for 1 man operations.
I disagree with the number 100. It is not a reasonable number with respect to current AI capabilities.
If the cost of digging ditches goes down, companies will be apt to dig more ditches. I've seen many projects worth $100k-$500k but they weren't pursued because the cost in salaries was higher.
I don't think companies are generally well run to let go of unnecessary people efficiently. At least one place I've worked at had, by my estimates, $100m salary of unnecessary people employed. It didn't matter, the business has revenues of $100b yearly, so it's a drop in the bucket.