I understand your points but why envato would be responsible?
When designs are not leaving my profile and there are almost no chances whatsoever of copyrights being violated as I’m the creator of a design and all of its coded versions would also be uploaded form my profile but will be managed by respective authors under sub ordinate accounts for all the collaborations.
All the issues you’re mentioning I believe happen when a file is wide spread on different accounts and sole creator of the file is left alone behind.
As far as every design being different and each agreement having different terms is concerned that would stay the same as it is right now, the whole idea of this upgrade would be designs being owned by the one sole owner and uploaded from the same profile hence no copyright violations and the sub ordinate authors would only receive their agreed sales share as per the agreement between the two parties.
I still wouldn’t disagree if there are any legal issues involved but those can’t be of very serious nature and I doubt it because it’s just like someone is working on your wordpress website not as an admin rather having a junior or an account which has limited access.
For the payment/money transfer with in accounts, that might have some legal issues or some payment gateways issues etc but that’s also not something which can’t be implemented.
Envato is a rock star in online marketplaces business and if new features which support/help authors are introduced they’ll help increase the reputation and ease the process of partnerships on here.