From your perspective: What are the characteristics of a healthy career? Is it healthy to have a plateau where you sustain some level (say Sr SWE) and don't "advance"?

Yes that sounds healthy to me. I don’t think titles means much to my friend and he is just happy to continue learning (which btw is never ending in this technology landscape) and applying it to his job. He is just looking for enough annual increment to cover inflation and continued access to health insurance. But if he is not taking part in the mad rush and jumping up and down with excitement he is being marked for the next round of layoff.

It's only unhealthy for your career at that specific employer. Fortunately for tech workers in the Western hemisphere, you can always move between employers