The thing is, author’s have the right to remove their items any time they want, so if Envato were to force them to keep items on the site for a set amount of time then it would be a very different business model.
You need to keep copyright issues into account as well. If Envato kept any such item on the site for 15 days then they’d be partly liable for any issues that might arise as a result of that.
I know it;s a pain, I’ve had items removed from my downloads that I’d rather they hadn’t been… but I guess there’s not much we can do about it unfortunately. Best to make sure you back up any purchases and if you’ve not yet downloaded them, then you can contact support for a refund.