The trouble is that the system as a whole becomes a product that requires testing and bugfixes. When things are swappable, every system is unique and has only

The trouble is that the system as a whole becomes a product that requires testing and bugfixes. When things are swappable, every system is unique and has only been built a few times. Plus, it creates fragmentation instead of concentrating efforts on one project. It's easy to get situations like "I need feature F but that's only in B, we use A because C needs it". Use of any features outside the lowest common denominator quickly makes things nonswappable.