I like PM2 and it's successfully been keeping my APIs alive quite well, but how does the future look given the popularity of Kubernetes? What is the overlap like and what's not overlapping?
Fwiw, last time I asked myself this question my conclusion was that there was no additional value, and we opted to not use PM2. This was a long time ago however, so take it with a grain of salt.
Also, kubernetes cluster workers can fail. How do you want your workload to behave in that case? When your container is killed, should these services fail together? Do you want a better separation of concerns between your app dev team and your devops/middleware team (think disaster recovery and cluster maintenance)? Where does the app dev end and the devops begin? For other languages such as Java or C++, it would be out of the question to let these details matter to anyone but app devs. Their entire apps are in one docker image. Kubernetes should only manage instances of the app and the obvious dependencies that should be externalized (redis, memcached, postgres, mysql, etc.)
In my experience it's really a question of failure modes in your architecture and squeezing out small gains in performance that matter at scale.