To be fair, employees are no longer particularly faithful to employers-- after all, it's considered completely normal to spend only 2-5 years at a job, then move to the next one. This definitely complicates the benefits of on-the-job training: why waste 3-12 months training somebody to basic competence when they might leave a few months after that? Some of this is obviously due to employees fighting back against stagnant wages/lack of promotions, but it makes me wonder: who started the faithlessness first, employers or employees?

I don't know about started, it's been back and forth forever, but in recent history a big epochal change was the hostile takeover movement of the 80s. Many companies, including those that had had a reputation for loyalty to their employees, were bought out by corporate raiders who often took actions that lead to huge number of layoffs.