All the above is true about LTN (specially being a dump!) but it suffers from a couple of (other!) disadvantages in this respect.
There are good bus connections between LTN and LHR, and rail between LTN and LGW, which would tend to draw traffic away. The type of passenger who goes to the Indian subcontinent is more likely to be price-sensitive than time-sensitive.
LTN offers very little in the way of inbound or outbound air connections. Once it gathers critical mass and offers these, the picture will change, but I think there's a long way to go.