[woodruffw]

I'm not confident in this, but: if microfilm/fiche is anything like ordinary film, it's somewhat annoying and costly to archive: the film itself physically degrades ("vinegar syndrome"[1]) and might be hazardous to store in bulk (depending on the age and type of film stock).

(This isn't to say that it should be thrown out, but that the first step to archiving is to enumerate and cover the costs.)

[1]: https://en.wikipedia.org/wiki/Cellulose_acetate_film#Decay_a...

[Scaevolus]

Microfilm doesn't use cellulose acetate, and is stable for centuries with minor storage requirements (room temperature, low humidity)-- the same that are required by books.

[woodruffw]

Modern microfilm doesn't; what about the kind that's holding newspaper archives from the 1920s?

[dredmorbius]

It seems that cellulose nitrate film stock was phased out by the 1950s:

<https://www.nedcc.org/free-resources/preservation-leaflets/6...>

How widely it was used prior to that time, and what filmstock selection practices specific institutions followed I don't know, though I suspect these may have varied.

As for current polyester filmstock:

Black-and-white polyester film has a life expectancy of 500+ years under proper storage conditions.

(From the same source.)