集體回憶 and 保育 are such overused terms in HK nowadays, to the point of becoming cliches.
That said, things must go on, as much as I am sad about it, we cannot expect KMB to do something about it...
There are even older buses of the past that have not been preserved, otherwise, I would say the case is strong enough that some kind of a museum, maybe a HK Transportation Museum, could be established. At 10k (US) ea these old buses are surprisingly affordable, and not too expensive to main either.
Perhaps KMB can donate an example of each type and some sort of authority be established to run it. The financial impact on the benefactors, especially if the government is involved, seems minimal.