> How many man-hours go into various parts of the advertising distribution chain?

Not that much. AI is already heavily automating advertising and has been for a long time. And a lot of activity that was previously exposed to untargeted passive advertising - like TV - has shifted to mediums that don't have any, like Netflix (for most subscribers).

And as you note, advertising isn't useless. It's how people find out about things they didn't know they wanted.