That would put those who are good sneakers at a disadvantage, also everyone who can't kill stuff in level 250 regions. Besides, the system even now allows for some luck. As TalkIRC can confirm, he once got an exploration achievement seconds prior to being mauled to death. Had he arrived just a bit later, he would have had to redo all the trekking and surviving.
tl;dr manual triggering of achievements would still be a good idea
tl;dr manual triggering of achievements would still be a good idea
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