Business is strong; load factors are good; geographical location is good; there are a number of major cities left in Asia to fly to. I'd say they know what they are doing, and that Asia - Europe channel clearly has the potential to fill A359s on the main routes.
(I'd ask whether A350-1000s should be ordered instead, or even A380s, but again the guys probably know what they are doing. A small subfleet would complicate things, even if on routes like HKG or PKG there'd probably be more demand.)
Anyway, this whole topic is one of those a.net dilemmas -- should one have more seats or less seats? With the enthusiasts drawing lines, often between the 777 and the A380 where the profitability ends and you have to sell tickets for a bargain. Now, in reality that's an issue that every airline manager needs to deal with, whether they fly E-170s or A380s or something in between. Rightsizing their offering. However, I think the OP took the wrong approach by assuming that "things will stay as they are". There's no natural "AY market" that has a set number of passengers. It is a competitive market, on price, but also on time and availability of flights. I think it is a given that if you fly efficiently between major cities in Asia and are able to connect to most capitals and few other cities in Europe, there's plenty of market to capture there. Not infinite market, but also not limited to 787 or 338 size.
The options really are limited for European passengers. Either there are direct flights, which is true of places like London, or you have to change planes somewhere. But you don't necessarily want to fly to London for instance for several hours and then back several hours, because that's wasteful of the passenger's time. So AY is in a good spot, particularly with SAS being mismanaged like it is. And Berlin's airport woes... you don't need to be brilliant to succeed in this environment
DY's longhaul plans will make things very interesting though
(Of course, not everyone likes AY's arrangements. Maybe someday BA will buy AY out of the way, and shut down operations to move more stuff to that horrible disaster-in-transit called LHR. But all this is just my opinion.)