Its not a matter of infinite casualties, its about the scale of casualties.
If the Federation lose 23 MI thats a significant loss of man power in the region, as they operate in incredibly small numbers and have to be shipped over from federation space.
Conversly lets say you have a warrior bug heavy army of say 200 Warriors and you loose them all. Is that going to affect a hive? Not in the slightest, its an insignificant figure.
The federation has access to orbital bombardment, tailored viruses, and god knows what else all the super high tech wonders of a technologically advanced human empire.
The bugs have near infininite numbers, thats it.
Thats what SST is about, if you think that means humanity can't win and isnt true to the book, film, cgi series, computer games, board game and V1 TTG then I really do wonder...
In the book/film do they win by beating all the bugs? No they don't they win by taking out the command assets in the face of the endless tide of space communists!
Shatter is a silly rule for SST, its only purpose seems to be a quick and easy introduction to the game until the main rulebook is released. And thats fine, for new players it probably does serve a purpose because its a gradual slope into complexity, for BF:E it makes sense, hell for the MI it almost makes sense, but for the bugs it doesnt.
But it doesnt really matter because anyone who feels that way is just going to ignore it anyway until the proper rules come out.