Also, it's weird to casually call people becoming homeless and starving and dying due to lack of work "slack". I'm sure the people desperate for money see that as a crisis more important than whatever abstract system crisis you are imagining.
A labour market should have people dropping out for having babies, going to education, sickness, looking after elders of the family, going on adventures and self improvement, for example people that want to write their own app/book.
Attempts at full employment were tried, it resulted in dead economies. This stuff is basic!
Socialism doesn't require forcing programmers to be hairdressers. That's a huge strawman.
For the rest, sure, there are some who search but cannot find work. But what that usually means is they can't find work that matches their specialism. Maybe they could go stack shelves for a bit, but if your prior work was as a senior lawyer it makes sense to take the time to find a job that matches your skills.