This is called the "lump of labour" fallacy:

https://en.wikipedia.org/wiki/Lump_of_labour_fallacy

But a jobs worth of GDP was lost due to the lost consumption. Harder to measure for 1 person but imagine 100k people suddenly left a city. That would be felt somewhere. Dry cleaners, cafe, supermarkets etc.

This might be less true if there is resource starvation but we have transport and imports and exports. You can accomodate more people and feed them.

There are not enough qualified people in any particular country for all the possible new technologies that could be deployed. You're not likely to hire your plumber to program a webapp.

That doesn't mean your plumber isn't qualified—just that people looking for webapps want to hire workers who know how to make them.