What's the core trade-off microservices make compared to a monolith?

You trade deployment/scaling independence and fault isolation (one service crashing doesn't take down the whole system) for massively increased operational complexity: network calls replace in-process method calls (with all their failure modes — timeouts, partial failures), data consistency across services becomes eventual instead of transactional, and you need service discovery, distributed tracing, and API gateways just to operate what used to be a single deployable.