If AI lowers the need to hire people by x% for a given project, then it doesn't just affect existing projects/companies. It lowers the barrier of entry for running such things for everyone. This means more people will be able to run similar ventures, and they all need the (100-x)%. In the long run, the amount of work available is not lower by x%, it could be lower or higher in general, although changes definietely happen, like in any disruption.