I'd argue that just about every infrastructure that looks like this benefits from Kubernetes (that you're not setting up and managing), and that's a lot of them. The biggest problem is that not enough people have boiled down Kubernetes enough to look like heroku yet. Google Cloud Run is possibly the best example of what Kubernetes can look like/run like -- it runs on (probably a relatively heavily modified) KNative, a project that runs on top of kubernetes.

The "point" of Kubernetes is to drop the difficulty of building a service like Cloud Run to zero. It drops the cost of building a Heroku down to zero. I'd bet my bottom dollar that fly.io and render are running on Kubernetes (maybe they mentioned it somewhere already and I just missed it). With the right cluster set up, building one of those platforms (or others that I won't mention) is almost as simple as setting up stripe checkout and writing a web interface to turn form fields into JSON fields and send them to a kubernetes cluster (possibly with hard multi-tenancy! not to get too into it, but you can literally provision kubernetes clusters from kubernetes clusters, ephemeral or otherwise).

No other tool in the devops world except for maybe the initial orchestrator wave (ansible/puppet/salt/chef) has been this much of a force multiplier. Ok, maybe that's hyperbole, but if adhoc-bash-scripts->ansible is 1->2, Ansible->Kubernetes is similarly 1->2, especially if you consider baked in cloud provider support/innovation.

But here's the secret -- perversely, I'm happy deep down that everyone thinks k8s is too complicated/is a buzzword/isn't worth the effort. All it means to me is that I'm still ahead of the curve.