dunno why co-op went from being standard from DOOM to the whiny "we cant doooo it" excuse it is now. all other aspects of FPS have continued to survive and/or improve. if game dev is anything like application dev*, then what usually happens is the heavy lifting was done once/long ago (DOOM1), then added to & improved upon w/ each release.
* and i suspect it is, after reading a carmack quote that some of the orig Quake 1 code is in Half-Life 2. a lotta code is copy-and-pasted from one version of an app to another. and if not, then the general idea or workflow is copied but the code itself re-written. so, im thinking co-op disappeared because 1) it was omitted from many releases a few years ago, 2) new devs didnt know how to re-implement, and 3) level designers began designing co-op un-friendly maps (collasping tunnels and whatnot)..
#2 should be addressable, if they look at older releases source code. and for #3, as Serious Sam showed us, creative solutions can be had, such as the warp-to portal which appeared w/ a stray co-op player became isolated from the rest of the group.