So is the suggestion above in your own post not another niche fare?
Clearly not, as the vast majority of passengers travelling between Birmingham and MK/London on LNR/WMT Only fares at whatever level will do so on a direct train via Northampton. British people don't like changing trains, and a WMT guard above has confirmed that, unsurprisingly, hardly anyone does.
Journey planners may show the connection as being faster, but will generally also, for competition reasons, show the direct even if slower.
The LNR/WMT Only Super Off Peak is fairly niche (it's only useful for evenings out and Sundays given the swingeing restrictions) but I'm talking about all LNR/WMT Only fares including the Anytime and Off Peak.