I am afraid I just can't agree with you people here... Connacht was the first person in a thousand years to defeat the Myrkridia in battle... that means that the war itself had been going on for a thousand years... even, as you say, it was just little roving bands of Myrkridia for the first 999 years or whatever, those roving bands would still have to have won every battle! I find it hard to believe that humanity survives a thousand years of war during which every army that challenged the myrkridia gets totally wiped out.
As for Myth2: They dont get all resurrected at once, surely you played the Tain level in Myth2... the Summoner is "slowly" (quote from journal entry) resurrecting their race, he is bringing them back one at a time.
Now, if it was simply a thousand years of war, I might buy that... but a thousand years of war during which humanity never wins? Now that I am thinking about it, it COULDNT be small bands or whatever, because if it was humanity could have won at least one battle. It had to be whole armies of them for the armies of the Cath Bruig to suffer a shut-out, utter defeat, at their hands...
Do ya really think the krids are really gonna sit around and say, "Well, we are a bloodthirsty race of super-strong cannibals who have been batting a thousand for a few generations and have not even come close to being defeated, but lets wait another few hundred years before wiping out humanity... better safe than sorry!"