Well, at least in my limited experience working in product management, the metrics I have seen is any additional step will increase the drop-off
Your assumption is that people who came to AC website already want to purchase a ticket, I dont agree with that belief. My take is that there are quite bit of browsers who are just casually checking out the ticket price, or that they may simply be shopping around. Introducing a login process before the payment page is counter intuitive and remove a % from your top of funnel that might have went through with a purchase.