Cool analogy Im gonna use that.
Only this morning I was trying to convince a customer exactly this. An old system is kept serviced is fine, all the while the customer doesnt need to make any changes. In my instance he had to change from digi to LAN coms. I tried to expain that its not worth bolting on kit to an old panel, because when the old panel dies you end up spend twice. Once on the bolt on, then on the whole upgrade. If you can afford it its always better to go for the upgrade in the first place.