I have downloaded, used, and recommended “pre-built” vaults before; Example workflows for Obsidian and the Dataview example vault come to mind. Both are great and I learned a lot.
The biggest issue I see with something like this (and it happens already) is folks downloading an elaborate vault full of complicated plugins hoping to have the perfect setup for XYZ but not understanding how vanilla Obsidian and the plugins work or interact. They are going to hit a wall sooner rather than later and have no idea how to troubleshoot the issue because they’ve spent no time reading the documentation or setting the plugins up fresh themselves. You have to put the work, or at least the time, in.
I think adding to the Hub site Cawlin linked or a new one collecting ready-to-go vaults could be a good resource.
My 2 yen. 