When should a .NET application use a modular monolith instead of immediately adopting a microservices architecture?

Microservices can provide independent deployment and scalability, but they also introduce distributed-system complexity, networking concerns, observability requirements, and data consistency challenges. A modular monolith can provide strong separation of business capabilities while keeping deployment and operational complexity lower. This discussion explores factors such as team size, domain complexity, deployment requirements, scalability, service boundaries, and organizational maturity when making the architectural decision. Understanding when not to use microservices is an important part of becoming a strong .NET solution architect.