Is 90% of system design only relevant to service-oriented-infrastructure / microservices? What about monoliths?

I recently learned about system design (from here). Stuff like scaling, load balancers, message passing, decoupling components, and BASE. I struggle to understand how these are relevant to single-program monoliths. I can only understand most of system design in the context of an AWS cluster. Is "system design" only worth studying if you intend to develop non-monolithic software? What aspects of system design (besides for best-practice concepts like SOLID) I apologize if this is a repetitive question, and thank you.