Sacred flame does get an "upcas"t upon reaching certain levels, from the PHB :
At Higher Levels. The spell’s damage increases by 1d8 when you reach 5th level (2d8), 11th level (3d8), and 17th level (4d8).
From the meta build and reviews I found online talking about this cantrip (
here and
here), it seems to be considered as one of "the best source of radiant damage in the game", because of how it scales when leveling up.
As the damage scales, it will easily outpace your damage with a weapon, so by 11th level there is usually no reason to keep a weapon in your hand.
And since, it's a cantrip, it doesn't cost any spell slot!
