There is no reason why cross border travel would become harder. As we will see, whatever works for NI/ROI will be adopted on this island. There is no reason why something that works cannot be replicated.
Even if, for some unknown reason, Scotland resided outside the CTA, there are plenty of examples of sleeper trains crossing frontiers all over the world. It may involve a stop at the border, but I can't think why that would put off the vast majority of passengers as if that were the case then any travel between Scotland and England would incur the same checks. The train would be timed to take that into account and the process would be implemented.
Having worked at the airport, before covid anyway, I can assure people here that Scotland, especially Edinburgh is a huge destination in its own right. Yes, there are a lot of people who add Scotland to a London itinerary, but at one point the airport saw 5 North American flights, 3 Middle Eastern Flights, and dozens of European destinations every single day. Those planes were almost always completely full in peak season, the airport could barely cope. Even during the winter the New York, Doha, and Dubai flights were always very healthy on a daily basis, in and out.