That's how it used to be.
It was taken off, because it's not realistic; in real world, an airline based in, say, Heathrow and flying to JFK and Tokyo, can't open a route between JFK and Tokyo because the governments of USA and Japan don't want airlines from other countries to compete with airlines of their own. The same goes to any country, except for inter-EU and some exceptions.
In the real world, it does require the 5th freedom between the external two countries. But there are many examples of that, e.g. SQ operates SIN-HKG-SFO, CX operates HKG-YVR-JFK, VS operates LHR-HKG-SYD.
I just don't know why this is taken out when multi-bases was allowed in previous games.
More realistic, in a domestic market, there is no restriction for this kind of routes. E.g., in China, it
is very common in have CAN-xxx-PEK where xxx can be any countries. You may say that by having multi-bases
this kind of routes can be "simulated" easily. However, opening a new base costs a lot. It doesn't make sense to open a base in an airport where demand is less. But by having ABCBA route, we can serve that airport easily. And this is what the reality does!!!
There's only one case that is not allowed in the real world. An airline that does not belong to that country (or EU) cannot operate domestic sector in that country.
I think we can just make an restriction to that case (the last one), while re-open the ABCBA route to other cases (e.g. three countries, or fully domestic).