Yes darkness is one of the most 'complete' power decks that can stand alone.
I think the cap was for more than just fire stall, it was for some types of fractals, and yes to weaken darkness.
When I started playing, before I even viewed the forums or wiki and was just figuring out the game, after a trial run at the game I thought darkness was the best starter element and reset, then built my first AI3 grind deck. I maintained a 100% win rate with this against AI3 with around a 70% EM rate (knowing what to steal is important as some elements were close games if you started stealing pillars when you should of held for a phase shield, or knowing they didn't have a powerful perm and just steal to add to denial/stall effect:
Hover over cards for details, click for permalink
5uk 5uk 5uk 5uk 5uk 5uk 5uk 5uk 5uk 5uk 5uk 5ul 5ul 5ul 5ul 5um 5um 5um 5um 5uo 5up 5up 5up 5up 5uq 5uq 5ur 5us 5us 5us 5us 8pt
and I used another mono unupped darkness which at the time had a 50% win rate against AI4 and is what I submitted in the unofficial competition on this forum.
I think that's a bit overpowered, the nerf, has taken about 10% off each of those deck's win rate, to around 90 and 40%, which makes more sense for such cheap decks.
So Firestalls and Darkness... I've never played much water, but water has a lot of power in it... but some fractal oriented deck's are also slightly hurt too, as some someone said, gone are 8 dragons at once, a slight shift, but one that at times can mean a win or loss...
So I think it was intended for more than just firestall.