Comment by Jtsummers

Good modular design is helpful here. Properly segregating responsibilities means that portions of your code base can become "finished", while other portions remain in near constant flux. For an emulator, for example, if you separate the rendering from the hardware emulation portion, you can leave the hardware emulation portion untouched for years at a time while changing just the rendering code to port to new platforms.

The best "old" (30+ or 40+ years) code I worked on did this. The worst, which forced total rewrites, mingled everything together "for performance" but prevented the software from being easily ported to a new OS (Windows 3.1 hasn't been supported for a long time) or extended to support new capabilities.