I understand quite well how monophyletic clades work and I hope you understand that they are a human construct. That doesn’t mean they are wrong or bad or something, but the question you are ignoring is: Who decides what the stem dinosaur is and would the decision be the same if they know about pterosaurs? Maybe but maybe not. Now that decision is made and set in stone, there is no going back but “dinosaur” and “ornithodiran” are just words that would be swopped (again, not gonna happen anymore)