Maybe it will hurt them, but it may not as well. Tech, for better or worse, is moving so fast that working on the same codebase for 5+ years is likely to great

Maybe it will hurt them, but it may not as well. Tech, for better or worse, is moving so fast that working on the same codebase for 5+ years is likely to greatly reduce your marketability for a new job. While that loyal individual may have been very valuable to his former employer, much of what he learned/specialized in is specific to that company and provides little value to someone else. One can always blame the individual for not spending his free time staying current, but not everyone is at the life stage where they have the ability/energy to end a 10 hour day at work by coming home and spending another few hours coding. Personally, I agree with you someone with a string of 18 month jobs would put me off as an employer. However, if you find yourself stagnating in your job, you must move on.