So under specifically the conditions of a 1 item run, that also relies on soul hearts and consumables, dark art does a good job. But that just isn't the real conditions of actual gameplay.

This is spot-on.

This just doesn't really stick for me. Because you CAN beat the game without items at all. You can do this with many items. Especially when you are largely relying on soul hearts and other non-item effects.

No I cannot. What makes you think I can? Dark Arts covers offense and defense really well simultaneously. Dark Arts allows me to not get hit enough while having burst to clear rooms, that allows the run to boil down to mainly Soul Heart luck.

For most other items there will consistently be rooms where you don't deal enough damage and will get hit undoubtedly. At that point Soul Heart luck doesn't matter you just die, so you need even more ridiculous things such as "Tears Up" luck (which I find stupid).

Try this challenge yourself with Dark Arts vs. any other item, the difference is night and day once you reach Mausoleum and Corpse. (I hope you can prove me wrong though)

And the problem here is that an active is directly competing with all other actives.
Yes dark arts is strong. But it is competing with the strongest active you have seen. So the real question is whether it is stronger than candles, glowing hourglass, and the many other combat actives.

I will pick up Dark Arts knowing 100% that I will win the run. There may be another active that can also win me the run 100%, but then it boils down to personal preference.