The main reason modular keyboards don't sell is the same reasons why additional peripherals don't sell. if its unnecessary luxury then people won't buy it, if people can adapt they won't buy, if it costs more people won't buy, if it is not standard people won't buy.
Its hard wired into people's heads to follow each other and the point is getting a result from the purchase not the sentiment that comes with it.
Only clothes and popular expensive items like cars, cellphones and mp3 players come with sentiment attached. This is the life of all replacement societies why spend more when you can replace it.