*sigh* no ESO will not kill WoW...why? read here: http://www.gamesas.com/topic/1492519-success-failure-and-expectations/
before every mmo release since WoW people claimed that "This game will ruin Blizzard" nope... did not...and ESO will not be different. Don't get me wrong... while I have a few complaints I think ESO is a good game... different from WoW and imho better...
But let's look at SWTOR... far bigger franchise... great developer...huge publisher...did the release make a dent in other game companies stock? Nope!
Another reason is... just because a game is release does not mean the markets react... the market reacts when Blizzard or any other company announces subscriber numbers and quarterly earnings.
I would actually recommend buying Activision Blizzard stock (Blizzards parent company that is actually traded) because Reaper of Souls... almost guaranteed success... should make the stock jump a bit.... might be to late though...