Early-stage SaaS teams usually need fast, safe product change more than independent service deployment. A modular monolith is often the strongest default.
Distribution has a price
Separate services introduce network failure, deployment coordination, tracing, data ownership, contract versioning, and more operational surfaces. Those costs can be worthwhile, but they arrive immediately.
Modularity is the real requirement
A monolith can still have explicit domain modules, owned data access, stable internal interfaces, and dependency rules. Those boundaries improve change today and preserve extraction options later.
When a service earns its boundary
Independent scaling, strong security isolation, a distinct reliability profile, separate team ownership, or a workload that needs a different runtime can justify separation. Use observed needs rather than projections without evidence.
Plan the extraction path
Keep integrations behind adapters, avoid shared database access across domains, document events and responsibilities, and measure workload. If a boundary later needs independence, the architecture will contain useful seams.