This has a flywheel effect where once you start gathering critical mass in a place, the original experts lose their edge eventually. See what happened to manufacturing, where China is leaps and bounds ahead of Western nations.
You haven't seen any european factories did you? They are proper high-tech in comparison to your average chinese one.
What people might not realise that manufacturing is still going on in Europe and US. It's just that less people are working in factories. Output is 'more' than in decades before. Automation and all that.
Lot of that employment is under-employment like Uber/Doordash/Instacart contractors and nowhere close to well-paying manufacturing jobs of the prior generation
OP said "if you're in a niche and have deep expertise you'll be fine". The analogy apparently means that deep expertise in a particular field won't guarantee your employment when that field is not in demand anymore.