Downsides (see: tremendous complexity) being generally understood, I'll list some of the upsides I personally feel compared to your described setup:
- If you wanted to scale up or down the number of container processes running on your VM, you'd need to write some code that looks at system utilization. Autoscaling k8s clusters do that for you. They can even provision additional VMs ("nodes") for you during times of heavy traffic, or scale down to save money.
- Updating your app requires either logging into each VM, or writing an Ansible playbook to do that for you. By the time you've written a zero-downtime, health-check-honoring, contextually-aware Ansible playbook, you've made your own container orchestration solution.
- If you run multiple containers that need to talk to each other, you'd need to handle their networking. K8s gives you tools for handling networking between containers in the same namespace that allows them to communicate without exposing them to the wider internet.
- The ecosystem of utilities is as good (and sometimes better) than you'd experience in your VMs setup. cert-manager makes certificate management almost as easy as LetsEncrypt does on a single machine. Prometheus and Grafana are excellent logging and monitoring solutions (and, IMO, much easier to setup on K8s than ELK is within a distributed VM setup). Cillium provides extremely powerful and useful networking and security policies that leverage eBPF
- Changes you make to the configuration of your server won't carry over if you ever need to switch hosting providers, or (more often the case for me) just want to start fresh.
It's absolutely a huge learning curve, but eventually the complexity (mostly) goes away, and you're left with a reproducible method for deploying apps. So in the same way a rails/django developer might use an overpowered solution for their blog API, or a React developer may build a custom frontend when wordpress would also do... someone who's taken the time to familiarize themselves with K8s might find the familiarity and consistency of the interface enjoyable, even if it is clearly killing a fly with a sledgehammer.