I'd say modern API deployments are either containerised or go the serverless route.

On one hand you have a docker image with your source code/static binary baked in. The other you have your source code/binary encapsulated in a serverless framework (with an Event handler of sorts as your input).

The container doesn't lock you into a particular ecosystem, but hasn't solved how you deploy and run your container (Heroku, managed k8s, AWS ECS/EKS/Fargate). Serverless on the other hand will give you a runtime for your API and allow you to go live (assuming you've setup an account or whatever).

To migrate something that already works to serverless is a bad idea. Maybe if you're pretty comfortable with serverless frameworks and you like to hack a lot of projects up quickly, sure serverless might be a good fit. But why not instead start off with a container? At least you have the flexibility to run that on whatever you damn want - Even a t2.small instance with a docker daemon running.

Heroku gives you a runtime environment also, using Buildpacks. In that respect it's already "Serverless".

Because most people with strong opinions aren't experienced enough to justify them from direct experience. Instead we have religions, tribal affiliations, camps.