Hello! Reviewing your idea and wondering if this is what you're speaking about? On event create you can select all with the checkbox under the header name of event setup. 

I tried to do this a few years ago and we ran into some blockers when an event was a hybrid event. But we could probably give this another go in the future. 

This is actually possible right now (as long as it's not with a package). They just navigate away from their cart. They still have to pay within the 15 minutes though. But yes, we should make this more clear.