Lots of things aren't cloud native applications. Most of the internet still runs on random C, Java, and PHP apps happily humming along on a handful of servers.
They aren't really the same type of product. Envoy is mostly an alternative to something like HAProxy, or Træfik. You wouldn't pick Envoy over Nginx (or Apache) if you need a webserver, than wouldn't make sense. Envoy solves a very specific problem, Nginx solves a bunch of problems.
Also if you're using Nginx everywhere else already, you might want to use a product you're comfortable with, even in the cases where Envoy could work for you. E.g. we use Apache almost everywhere, because performance is "good enough", and it allows us to use the same product for a lot of different client, due to the large feature set.
nginx is a reverse proxy and app server. It has all the major features to handle front/edge tasks (like TLS termination), serve static files, and run entire applications in-process given the correct modules. nginx has entire ecosystems around this with OpenResty and even runs the edge workload for Cloudflare's CDN and Algolia's search service.
envoy is a L7 proxy focused on full duplex networking across multiple protocols with transparency and observability. It can also handle front/edge tasks although the configuration system is a magnitude more complex than nginx and it's also missing the app server abilities.
If you use envoy already then it might be easier to deploy it to the edge as well, and it's often used that way in Kubernetes as both service-mesh and ingress - but if you just need typical reverse-proxy/app serving than nginx is still far simpler to setup and maintain.
