Your argument essentially amounts to "this is how it's worked in the past (unemployed people have been able to find other categories of jobs), so it'll always work that way".
When labor is freed up, entrepreneurs look for new ways to employ it to make money. When production costs are lowered, entrepreneurs look for new ways to employ the freed up capital. This is why so many companies are hiring at the same time as they are firing. Taken at a national or global scale, the result is that labor and capital follow supply and demand curves and that adjustments to the system through automation are compensated elsewhere through economic incentives.