These B789s will be initially used to replace TK's aging A330-200 fleet from 2023 onwards as the oldest A332s were delivered between 2005-06.
Though no configuration layout has been revealed, it is expected that TK's B789s would seat 291 passengers in a 2 class layout i.e. 30J + 261Y. With this cabin configuration, it is also the ideal replacement aircraft (size and range wise) for its A333 fleet between 2025-28.
The B789 can fly all of TK's current routes nonstop with a full payload of pax + cargo but for it to launch nonstop services to SYD/MEL, it will need to either order the Boeing 777-8X or have a less dense B789 cabin configuration (with more J class seats similar to what QF is going to use on PER-LHR effective March 2018).
This is a very smart decision made by TK as:
1. The B789 would likely seat 291 pax in a dual class layout i.e. 30J + 261Y thus enabling it to be used to replace both its 250 seater A332s and 289 seater A333s between 2023-2030.
2. The B789 can operate all of TK's current long haul routes nonstop without any payload issues
3. It is cheaper to purchase versus the A359
4. By placing an initial order for 40 units, it has smartly locked in favorable pricing conditions as this aircraft will appreciate in value over time hence once it gets delivered, TK can engage in a sale-leaseback transaction which would have a decent cash profit margin for the carrier.
5. Hopefully this would mean that their future WB fleet would revolve around 2 types only i.e. B777 and B787 rather than 3 i.e. with the A359 included as there is now no real need to order the A359 for them. Fleet standardization is a must which will enable them to save big time in cost savings over the long run.
6. I am quite sure TK would have options for the larger B781 incorporated into this order in case they feel the need to upgrade for some high demand medium range flights. The B781 would likely seat 329 pax i.e. 30J + 299Y in a 2 class layout for TK (if ordered) so about 38 more seats than the B789 overall approximately.
For sure this has gained the Turkish Govt major brownie points with the Trump Administration for which they will be suitably rewarded through other means!