I believe it’s already on their to-do list, but it could be a long time till something like this is implemented. You can create your own plugin or function which will pop up the alert message in wp-admin once their theme has been updated.
I love your ideas kid!
and yes, it’s been suggested and it’s on the to-do list. I have no idea when an item has been updated and have had problems before with some items I’ve purchased and then come back to the marketplace and find an update. I’ve seen some authors implement a subscription email list to receive updates. The “follow me on twitter” is another option but I understand it’s not the same as for example have a list of update items on the front page or something like that.