You are only refering FaaS here for serverless and your main concern look like vendor lockin . have you given any thoughts of using Knative where you can run your container as serverless and this should work on any K8s env by any public cloud provider. Regarding cold startup what is your thoughts on using graalvm AOT and then knative servless . this combination should give us cost saving and any vendor options.
the promise of serverless isn't really about cost as much as its about not having to manage servers, but cost is also a huge benefit when its used for the right workload. obviously you shouldn't use lambda if the workload requires on-demand compute because that's not what lambda is for. And i think one of the main reasons you aren't seeing much serverless architecture in your space is because you live in Java world and no one wants to see Java in a serverless microservice stack.
The part about java is not really true anymore. You can take a look at native image support where java applications can start without the virtual machine (quarkus for example). Even normal jvm applications can start very quickly with a technique called crac nowadays.
You’ve got the wrong end of the stick here. For really bursty/unknown traffic patterns, Serverless 100%. Lambda scales from one instance to 1500 in about a second. Fargate takes about an hour to scale to 1000 EC2 instances, from memory.
