Shared mutable state also goes beyond a mutable reference. If you call a function and that function throws an exception you are tying the caller/callee's states

Shared mutable state also goes beyond a mutable reference. If you call a function and that function throws an exception you are tying the caller/callee's states together. In a SoA the callee machine can literally blow up and your caller state is preserved. If your web service is generally low-state and these problems are manageable for the complexity scale you're solving for, microservices aren't really something to even consider - I mean, you basically have a microservice already, it's solving a single problem within a bounded context, give or take. It's just... one service and one context.