My husband and i just watched the most recent episode last night. I don't see why everyone was so stunned. I have seen the writing on the wall about Dany for a while now, at least from the beginning of season 7, and there were definite signs before that. She has been getting more and more ruthless and more paranoid and focused on the throne, not what is good for the people. I am not necessarily pleased that this is where the character has gone, but it was clear for a long time that she would reach a crossroads and have to make a choice.
FWIW my theory is that Aria will die killing Dany, and Jon will rule, but not as a king. I feel like its going to be more of a 7 member council, one from each kingdom, with Jon acting as the mediator. I don't feel like he will accept a crown again, but he really is the best person to rebuild.