[delusional]

>On the other side is the projected cost of keeping this thing and maintaining it, not just for this change, but for changes in the forseeable future.

That's going to be very difficult, especially when you then also have to consider that the changes you could do could REDUCE the cost of maintaining it. If you're only ever appending code, then the cost to maintain can quickly skyrocket. If you're also diligent about removing the features that you don't need, there should be no significant difference at the limit.

Basically, assuming you are properly maintaining the existing system, you should be continually refactoring it to be what you would build if you started from scratch. In that view, the cost of maintaining it will be identical, and the only cost that matters is the cost of bringing the existing system into alignment with what you would build now.