I agree that anything else is extra, but most of my difficult professional engineering decisions are tradeoffs between those first two things: I can build on an older system less well adapted to current needs and deliver more quickly, or build something that fits current needs better but takes a bit longer. I sadly don't have enough days in life where there's a better and quicker option!

Put differently, it's a lot faster to put another layer of duct tape on top of the last duct tape patch of a pipe leak. But every layer of duct tape you put on makes it more difficult to replace the pipe segment underneath. Developing judgement on when to do which is the (difficult) trick.

We deliver the duct taped one one time so the software folks can start their work, and then we build the non-duct taped one wheel it in a week later and quietly swap it out for the duct taped one.