A few things to note.
The old domestic terminal had already been approved for an upgrade to make it operational again - one airline wants to be the sole operator there. Indeed, work was meant to commence last year based on previous 'announcements'. Both the expansion of immigration and new air bridges have also been promised for sometime as has a new car park given the acuteness of that issue. Pending is also the APM which will link the 3 terminals and new car park and eventually run to the east side of the airport to link with the Air Force side and Phahonyothin rd/BTS extension. T1 also will be refurbished at some point (supposedly in the next 2-3 years).
Lastly, when the new super DMK 4 level suburban station is completed for the SRT Dark Red line, (ARL ext/Eastern HSR line and Northern/NE HSR lines) and opens in 2020, a new pedestrian link will be built to link the station with the southern end of T1. There's still dispute as to who has to pay for this, the SRT or the AOT - or shared.
As with all the airport expansion projects in Thailand, they are all running well behind pax growth. BKK is indeed 5 years behind in schedule, the Phuket expansion was perhaps 10 years too late. Forward planning is not exactly the AOT's specialty.....
Don Mueang studies airport upgrade , Bangkok Post, 11 March 2018
Don Mueang airport will propose a 2.2-billion-baht renovation project for its old domestic terminal to the Airports of Thailand Plc (AoT) for consideration next month, general manager Suthirawat Suwannawat said. The upgrading of the old terminal is part of the third-phase development plan for the airport, which is being sped up to ease overcrowding problems, said Mr Suthirawat. The number of passengers at Don Mueang is expected to grow by 14% to 42 million passengers this year, compared to the 37 million passengers who passed through the airport in 2017. If approved, work can begin late this year or early next year, and the terminal, currently closed to passengers, can be operational by 2020, he said. The airport management also plans to push for the implementation of another two projects to help the airport's passenger handling operations, says Mr Suthirawat.
These projects are the 22-million-baht jet bridge renovation project and the installation of more immigration checkpoints to increase handling capacity from 2,000 passengers per hour to 3,000 per hour. These two projects, alongside the renovation of the old domestic terminal, are being treated as priorities for the third-phase development plan of the airport. The third-phase upgrade is estimated to cost 32 billion baht and is expected to be operational by 2024.
Full:
https://www.bangkokpost.com/business/tourism-and-transport/1425958/don-mueang-studies-airport-upgrade