Arthur Blank wanted an outdoor stadium but once it was determined that keeping the Dome and building another stadium was not feasible, that possibility was basically out the window due to all of the events that will held at the new stadium. And I wouldn't call it a small opening because it is the size of the football field, but of course it won't feel anything like an outdoor stadium. On the other hand it will be a lot more open than the Dome ever could be. The design of the roof is already complicated enough to cause a major delay in its construction and increased cost of the stadium.
If an outdoor experience was the primary objective, they would have chosen the other option.