I'm not sure I buy that reason, unless the customization cost extends to the equipment and process for building out the interior. Surely this cost would be identified and then the options available reduced? But much harder, if your need to retool for a cheaper interior.
Around 1995, Boeing had a HUGE project to address this (DCAC/MRM). I wasn't deep in the project, but the stories I heard were crazy. Something like 30 shades of white paint, or parts that were functionally the same having two or more part numbers.
The one thing that has stuck with me is the statement that you can't undrill a hole. It was the same thing the parent poster mentioned: Every operator wanted their galley/lavatory to be in some Goldilocks location, and that required drilling holes. Boeing pushed hard to limit the choices in order to bring cost down.
Those holes also caused problems if a customer resequenced or walked away from a partially built plane. Southwest would want a discount if you were selling them a plane that was originally kitted out for someone else.
