Isn't that backwards...if common magic grows in utility relative to otherworld magic wouldn't the Glorantha of KoS be less mundane instead of more. I would suspect that a world where common magic, accessable without study/initiation into otherworld magic, became as powerful as otherworld magic we'd have a world similar to The Darksword Novels, where magic does everything (although I have a similar, but not exactly the same, image of the GL era...there magic is ubiquatous but as magical technology instead of magic outright).
Based on Greg's recent comment about the Gods War seperating gods, mortals, and demons from each other (as an aside, what distinquishes demons from other immortals) my theory is the Hero Wars will seperate mortals able to use magic (potential heroes, superheroes, and demi-gods) from other mortals. As a result, the end of the Hero Wars will result in either the destruction or apopetheosis of the last people able to wield magic. One could see the disappearance of Brithos with Zzabur as the first step in this process, making it the first defining act of the Third Age as opposed to one of the final acts of the Imperial Age.
With heroes and their ability to tap the otherworlds gone (the various heroes and potential heroes being the bridge between the mundane world and the otherworlds during Time), the mundane world will slowly drain of the magical energies that fuel common magic, disrupting all Gloranthan cultures and breaking them down (the illiteracy era) until people rebuilt culture on mundane foundations (restoration of literacy and beginning of the Harshax era).
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