First, the very concept of countless magic means little. And what kind of limitless magic? There are only 6 cosmological forces, and although there are various subtypes of known magic (like blood magic or chronomancy or voodoo), I don't think Archimonde has 100 different magic powers or anything, and that doesn't exactly put him on the level of a Murmur. Med'an had 3 magic at the same time, but Azshara would crush him like a fly. And I would also like to see a proof that Sargeras destroyed entire galaxies and so on.
As for the INFINITE UNIVERSES. If you read the book about Illidan, you would understand what it means. In his vision of the Legion, Vandel saw Archimonde destroying countless worlds in all universes because when he destroys ordinary worlds in the MAIN UNIVERSE, he destroys the POSSIBILITIES OF THE APPEAL OF THESE WORLDS IN ALTERNATIVE UNIVERSES BECAUSE THE LEGION EXCEEDS ALL REALITIES. Do you know how alternative lines work? This was described in Chronicle 3. They appear on their own reflecting different possibilities and different character choices, but if these lines are not connected to the main universe (as in Waters), these lines will disappear on their own. There is one true timeline and it was this line that was assigned to protect Nozdormu. https://wow.gamepedia.com/Aman%27Thul%27s_Wisdom
"Unto you is charged the great task of keeping the purity of time. Know that there is only one true timeline, though there are those who would have it otherwise. You must protect it."