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Runed Halo costs WW, while Ground Seal costs 1G.

But oh well, Runed Halo somewhat protects you against Storm and Belcher...

Uhm, you know about Ichorid and Loam? it cantrips on its own, yes. And the nice thing is that it gets protected from bounce by Sterling Grove, you can tutor it and it just has overall more synergy...

Against reverent Silence, you should smack down a Karmic Justice and start laughing when they wrath their whole board.

Seriously, why would you ever play a GWb list, when you could just run...

The thing is, that every other possible win condition seems to be better than luminarch Ascension.
Be it Sigil, Words of War or even Words of Wilding. Everything is faster and more reliable.