That has been the general trend over time, but AI breaks this trend. The basic problem is there is minimum wage below which humans can't survive. No such lower limit exists with AI. At some point every job can be performed for less by an AI.
Not true. The price of materials and the energy needed to run it determines the lower limit on cost for AI. The cost of energy to run computers is pretty low -- low enough that a lot of people don't really think about it until you get up to clusters of the size found in computer labs and data centers -- but it's not nothing either.
I will agree though that the minimum cost to run a super-human AI (once it becomes available) is likely to be substantially less than the minimum wage needed to pay humans to do equivalent work.
Sure some jobs will be replaced but new jobs will be created. Thinking on the spot now. A manager and a whole team of people that oversee the job that the A.I does. Jobs that focus on applying A.I to certain field, (A.I consultants)
People value human interaction and when there is value in something, there is money to be made.