I can't understand how you don't see the value in supporting ancient and dead languages with precise full text search, enabling lossless reproduction and alternative representations through printing, web sites, etc. Why wouldn't we want to try to keep our pre-digital records alive by immortalizing them digitally?
So it's been mentioned one of the big benefits here is reproduction and text storage to allow searching, etc. Another biggie not mentioned, however, is newly found texts. Yeah those languages are probably not growing and maybe nothing new is even being produced in languages we bring into Unicode but that still leaves the possibilities of finding texts previously undiscovered or simply not archived previously.
Regardless, I would prefer to have all characters of all languages, dead or alive, in Unicode so I can handle any new or old data that data set requires without modification.
Some of us like to read those ancient documents online in the original form, not in the multiple translations of it, some of them not 100% like the original meaning.
Our culture goes all the way back to the first days stories were told around fire in caves, in cold winter nights, before writing was a thing.
Everyone should strive to preserve mankind's culture.
There's no harm in it, particularly in further out plains, just my knee jerk reaction is "why?".