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.
uhm.... lotus 1.0 (or even 2.01) was quite well spread. try locating a copy :P