An internship isn't a magic bullet that cures all ills but it does improve thing meaningfully in several ways.

To address your point, I agree with you in part but giving people a chance who otherwise would not, does build loyalty making it more likely they will stay longer (on average). You still have to be a good company to have a chance of retaining people, it isn't just a cynical ploy to fool people into working for you. There is a middle ground between your example of 20-40 years vs 2-4 that is very meaningful because it takes a lot more time than people give credit, to get good at a job. So that >2 years time frame is very valuable.

I do think a lot of companies, but crucially not all, effectively treat even highly skilled labor as a disposable asset to leech off of. I also think an employment system that expects career advancement to require changing employers, is crazy shortsighted. Just as is degrading the public education system and putting young people into massive debt with college. The system has problems all over the place but an internship is a very practical way for a company to do better.