"It is entirely clear that 21st century Americans have no interest in jailing runaway apprentices. A legislative proposal calling for the arrest and detention of individuals in breach of their apprenticeship contracts would rightly be met with jeers and guffaws."
The big reason they don't like the German approach is because it excludes from the trades those who don't have the "appropriate" licenses. This shuts those of little means out of these jobs. They proposed, as possible solutions, vouchers, loans, and possibly permitting non-compete agreements.
Your accusation of "re-introducing slavery, or at least indentured servitude", is deeply dishonest.