It seems to be saying that you can do an awful lot of customization and it'll only make customers a little happier. It is only when you get to high customization that it makes customers much happier, but at that point you are well beyond sustainable costs.
Take men's suits. It's saying fully bespoke isn't a sustainable business model. Made-to-measure might or might not be (depending on which side of the line) but it isn't going to make your customers ecstatic, just a little happier than off the shelf.
One odd thing, is it seems to start by trending downwards. Not sure if that was intentional.
A certain amount of customization is cheap, easily supported, and makes customers very happy. A little more customization is very expensive and doesn't make customers much happier (quickly diminishing returns).
