When the Amiga system originally went in it was controlling well over 100 buildings throughout the district, including the entire GRCC campus at the time. The Amiga replaced the head-end of the system, which was experiencing expensive hardware failures every year
Each building has one or more local control systems, and those systems communicate back to the central head-end over radio-modem (there was no district-wide network back then). Schedule and other control changes are sent to the buildings and alerts/reports are sent back. That old equipment in the buildings, even older than the Amiga, is what dictates the radio communications link. They incorporate specific protocols for keying up the radio that are not directly compatible with a newer serial to Ethernet type device that would seem like a logical replacement.
The control systems themselves gather temperatures, both inside and outside the building, look at trends and do predictive control of the equipment to accomodate scheduled use of various areas of each building. For the day, this was very advanced building control and offered significant energy savings, as well as comfort in the buildings.
Over time, as buildings have been updated, sold or replaced, the local controls withing those buildings have been replaced with newer/more modern controls that communicate with newer central control systems. Replacing these controls that are local to the buildings is what is responsible for the majority of the cost I would say.
As far as the Amiga system itself, I believe most of the components are still the original. The hard drive may have failed twice over the years, requiring a rebuild from backups. They did pick up or have donated a few Amiga systems to use as parts as needed, but the system has proven to be very resilient. Obviously, Monitors, Keyboards and Mice can only take so much use without needing to be replaced. Without this, the system likely would have become inoperable and unservicable many years ago, or been incredibly expensive to keep running.
From a technical stand point, the Amiga was selected because at the time it was the only "Personal Computer" (PC) that had a true pre-emptive multi-taskng operating system. It needed to be able to handle multiple processes simultaneously, including interfacing with the systems, maintaining settings in the database, monitoring the system as well as support for both local and remote access to the system simultaneously. Basically, its capabilities fit the need. While for nostalgia reasons I would hate to see it go, it has been 30 years and I think the system has done its job. Replacing a building's control system doesn't happen overnight, and when you are talking 19 buildings with ancient (yes I am calling myself ancient I guess) control systems, it is going to take money and time. The payback in energy savings, comfort and safe control of the buildings though I think justifies the cost.
"The fundamental principle of science, the definition almost, is this: the sole test of the validity of any idea is experiment." -- Richard P. Feynman