if you look at society as a whole bigger things have been lost than either MAME or MESS.
Define "bigger". Duplication implies redundancy. If you want to make some sort of analogy to the libraries sacked in the Roman and Greek times, feel free, but somehow I don't think that's even remotely possible nowadays given the hundreds of thousands or millions of copies of the MESS and MAME source code spread far and wide across the world.
Given that libraries were razed to the ground during the Iraq invasion it's not just Roman and Greek times.
There are millions of copies (probably less of newer versions) which helps, yes, but they rely on a few basic fundamentals.
a) users having control over what runs on their own systems
b) users having control over what can be stored on their own systems
b) users being able to distribute such things easily
c) people actually caring enough to keep things in circulation
you're talking hundreds, not thousand or millions of people downloading the latest 'MAMERoms' torrents which is probably a fair indication of the number of people actively following MAME. For MESS that value is significantly lower.
Nowadays it's a struggle to find disk images of software which was widely pirated back in the day, especially applications. The interest in said software declined to the point where nobody was keeping it, and just because projects like MAME and MESS have revitalized interest in such things doesn't mean that interest is going to last forever. If you combine that with attempts to lock down platforms, censor the internet and the rest, then I don't see it as impossible.
Finding the old source archives for older versions even when MAME was at the peak of popularity was hard. I believe we have a complete untainted set these days however.
As long as the projects remain active it's less of a problem, however have you ever tried looking for the files for some older emulators? all roads end up leading to the same place, that place is often dead (old Geocities account, Domain squatter, or now MegaUpload link..) I've encountered this recently when looking for a couple of things to try playing with in the MESS 486 driver.
At their peak games like Quake 2 had HUGE communities of mods surrounding them. I played a lot of them. These days the files for many of the mods which didn't gain popularity are nowhere to be found, and huge numbers of unofficial maps are missing yet these were things that somebody would have painstakingly constructed over a few weeks, then spend days (in complex cases) compiling due to processor speeds of the period. I doubt the people making thought one day they'd be lost after they'd been spread around the community sites, and were running on a variety of servers. The reality is however a lot are now all but gone now with only the most popular ones remaining. When big hosts (like MegaUpload, Geocities etc.) pull their content the main victim is *old* content from no longer maintained sites. The people pirating modern stuff just reupload it somewhere else while older, forgotten stuff just vanishes for good.
Public domain libraries are no exception either, thankfully in the case of the Amiga a lot of the software ended up on CD-ROMs too, but everything?
All I'm saying is that anything can be lost be it physical items, or digital software. I don't see MAME as an exception to this. (incidentally this is also one reason why I'd prefer a combined project, MAME is only likely to become less active and have less people interested in it due to the natural decline of arcades whereas MESS has a more solid future)
How many places are actually mirroring the MAME source these days? How many are just pointing at MAMEdev.org, or only offering binaries? How many of those places are rock solid reliable sites you KNOW will still be around 20 years from now, with all their old files from now still intact? Most (if not all) of the ones from 15 years ago when MAME first started are long gone.