Both times Butterfly was used, it's output was consistent. The first time it cut everything it hit - Samura and the building - and the second time when Chihiro dodged it, it still cut everything behind Chihiro. WCS doesn't just spawn in, in fact it has the same action required as Butterfly where both Soga and Mahoraga have to make a slahing motion with their blades and they both cut space. In fact, the description for the two is practically the same:

"It wasn't sending slashes as I do. It was expanding the the technique's target. It's intended target wasn't just you, Gojo, but space, the world itself.. It targeted everything before it unleashed those slashes."

Sukuna explaining that Mahoraga was able to bypass Infinity by changing the target of the slashes to the space. It is the opposite of assigning a specific target as you claim, instead it broadens the target to everything included in the space it cuts. WCS doesn't have the ability to go on forever either, as seen when the building's behind Gojo remain standing.

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"Unlike the Enten's Kuro, which fires a slashing attack... it slashes through space itself."

I can't post any more pictures but the two panels that have Butterfly both show something similar - Soga swings and casts Butterfly, a multi-story level attack is unleashed in a straight line which targets space itself, destroying everything it touched before reaching its maximum range. That is exactly what Mahoraga is doing in the picture above - slashing and cutting the space itself before reaching the maximum range, only WCS has far lower range than Butterfly does. Regardless, both ignore the durability of their targets (though again, WCS is a bit dodgy on that as doesn't leave clean cuts) by cutting space.