Terminal 3 was opened in 2015, it is 7 years old since Terminal 3 started operation. Terminal 3 was built for LCC and the building itself was built with a minimal cost and maintenance concept, which does not have any commonality with other terminals. Terminal 3 does not have direct train station access, and also lacks convenient vehicle access to the terminal.
Terminal 1 North Wing and Terminal 1 Central Building renovation were completed in 1996, followed by renovations of Terminal 1 South Wing in 2006. Although Terminal 1 went through renovation about 20 years ago, the foundations and infrastructures of the building are from 1978 when NRT opened. Constructions of the airport itself started in the '60s.
Terminal 2 was opened in 1992 and is 30 years old already. Terminal 1 and Terminal 2 buildings are in need of constant maintenance, which is not cheap due to their age. Terminal 3 can use better access and lacks commonality with other terminals which limit gate use flexibility.
With all these considered, one option for NRT is not to maintain and rebuild each Terminal building separately but to build a totally new terminal that will handle all current Terminal 1, Terminal 2, and Terminal 3 functions. Looks like NRT chose this option.