What's frustrating about this argument is that the people who want to see Brexit succeed are jumping up and down saying 'it will be fine, nothing to see here' when the reality is if it is to succeed they should be jumping up and down shouting at the government to acknowledge this risk and sort it out on time.
With open skies, and many other things, the UK doesn't have any relationships with any countries in the world. Its not just an EU thing: there are no bilateral arrangements because the UK's arrangements have always been made as part of the EU.
This doesn't just mean all new agreements need to be reached: it also means that most of the specialists who negotiate this agreements come from other European countries (and in some cases the UK) and currently work in Brussels, because that was sort of the point - everyone didn't need to hire their own negotiators/specialists on various obscure topics, Europe was able to work together and cut costs.
Everything is eminently fixable - the problem is the UK government hasn't been putting in the effort needed to fix it. I know lots of people who are experts in particular fields who are horrified by how little effort, and how much ignorance, has been portrayed by the government on their issue areas. We see here its people from tourism concerned, but there are lots of other areas as well.
It is all things that can be addressed, but things that have multi-year lead-in times. Michael O'Leary loves his press and talking rubbish, but despite that, leaving things this late really does mean there's a risk aircrafts might be grounded for some period of time, there may be travel chaos, there may be food importing chaos - think of the snow and multiply it by 100. Imagine what Heathrow looks like when flights are cancelled for a day or two and now imagine what could happen if it was a week, or ten days, or a month while negotiations ran long. There may even be civil unrest based on how things are going so far with Northern Ireland. Alternatively, the UK may get deals on all these things in time but get really bad deals, because they've left it so long and they haven't recruited the right people, or simply don't have access to the right people.
So if you're a Brexit supporter - honestly, stop saying it will all be alright. Start pressuring your MPs and put pressure on people to act now so it actually might be alright, because while it may seem like scaremongering, there are enough signs at this point that a lot of things will be very messed up unless serious action is taken soon.