Well Venemoth isn't 4x weak to Stealth Rock, so at least in my book it's automatically better than Butterfree.
But Venomoth can be hurt by Spikes.
The only advantage I see Butterfree having over Venemoth is it's 90% Sleep Powder,
Butterfree has 97.5 Sleep Powder accuracy.
and I guess EQ immunity.
Not to mention SPIKES immunity.
Venemoth on the other hand has Tinted Lens, better stats all-around (better in every stat but SDef, which is 5 points less), Toxic Spikes, Baton Pass (with Agility), oh yeah and the most important one: Venomoth doesn't lose 50% of its health when switching in
Venomoth has a Stealth Rock weakness too.
To be honest I don't really see what Butterfree has over Venomoth. 30% better accuracy on one move IMO doesn't compensate for all of Venomoth's options, and its superior stats, and certainly not Butterfree's own Stealth Rock weakness
True, but Venomoth isn't COMPLETELY superior to Butterfree. Not as much as you make it out to be.