I agree very much with your comment. I've found the fundamental principle that helps to keep the system extensible is to: make your system model the real world

I agree very much with your comment. I've found the fundamental principle that helps to keep the system extensible is to: make your system model the real world accurately. This involves building the system concepts closely mapped onto real world concepts without taking shortcuts. That way when the requirements change, all the fundamental pieces of the system stay valid, and only the piece that is changing tends to need updating. This helps to void the problem you mentioned of needing to re-design the full system, keeping the system extensible.