Just because some of the design work may have been done already doesn't mean it is a wise decision from the start. Best I can tell from some sales records I found online; between Buick and Opel/Vauxhall etc they are moving <200k units per year. That is less than the number of Honda Accords sold in just the US. I would imagine the percentage of these sales that are wagons is pretty low. So that means they are making these modifications for a very limited production run. This can't be cost effective regardless of whether it is done for one market or 50.