Excellent question, one which has been delved into by the scientific community. Kosher slaughter results in Unconsciousness in 3 to 4 seconds, and is generally accepted as equally humane conpared to stunning or bolts, probably mor mumane considering the "error" rate of those procedures.
The first sensible response Ive heard in the last four days. If there is scientific evidence to show that it is more (or equally) humane to kill an animal this way, than to knock it out, then fine. Otherwise, why not adapt, at least seek a rabbinical opinion?