Yes, this is unprecedented.
Your problem is that you're confusing magnitude of the impact of disruption with the scope of the update. So you have line-of-business software powering your multi-gazillion-dollar business. If it's really that important to you, your tests for each security patch (which, again, have been known to break programs in the past) should be just as intensive as your tests after a service pack. There was a recent security patch that patched something in ntdll.dll--that DLL is loaded by literally every program on the system. If there were gazillions of dollars on the line, I'd test and double-test the hell out of that thing.
If Microsoft does their job and limits the compatibility impact of their changes--and if you are doing your job and doing thorough testing of updates instead of assuming that security patches are something to breeze through, then the difference between this and other monthly updates isn't as nearly as big as you are making it out to be.
I'm not sure what to say here other than you're not understanding the difference. Testing a single security patch takes far less time than testing a rollup nearly as big as a service pack. If everything goes well and there's no problems, then yes, it's an identical testing schedule. It's when something *doesn't* go well (like you pointed out, not an uncommon occurrence) where the big difference hits.
If that single security patch causes a conflict in your test group, awesome, you roll back, don't push the patch to the enterprise, and the devs engage Microsoft to fix it/work around it/etc. Or you determine that the risk of not applying that patch is minimal and you
don't ever deploy that update because keeping your LOB software working is far more important than some "wifi reliability" update when none of your devices have wireless cards in them anyway.
Here we have a rollup of 100's of minor patches and MS is
mandating that you install it to maintain support. Your enterprise doesn't have the option of just rolling back and not installing it, because when you have a problem with some other minor security update six months down the line MS is going to tell you you're up the creek without a paddle and they won't help you because you didn't install this other patch. If any one of those rolled up tweaks or changes causes a problem, you
cant just roll that one back, it's a package deal and your continued MS support hinges on it. It could take weeks or months to track down specifically which needle in the haystack is causing the conflict and now you're under the gun to drop all other dev and fix this patch that in all honestly is one you could legitimately get away with never deploying. You're essentially breaking the first rule of troubleshooting/debugging: only change *one* thing at a time between tests to maintain control.
You could argue that most enterprises aren't running 8.1 in the first place so it's a minimal issue, but the real point here is that they're setting a dangerous precedent by accelerating the support cut off dates for their flagship product.