The problem is that it is assumed that everyone wants to visit the centre of cities. Where I live it takes about roughly the same time to get to Heathrow as St Pancras. Getting to Paris by train wins out but Amsterdam is quicker by air.
In my experience advocates of high speed rail tend to be very passionate about their product and think that everyone should fit around it and prefer the stick to the carrot. Interestingly this is generally not the case with conventional networks where the carrot is more in vogue. Or possibly the car lobby is more vocal than the aviation lobby.
Environmentalists like to concentrate on a particular issue rather than take an holistic view. People need to look at their overall carbon usage but that doersn't fit the soundbite approach some (not all) environmentalists take.
The trouble with the French high speed network is that it is not very frequent at certain times of the day and having Ouigo as a separate brand doesn't help.
That said the nain route that will be affected is Paris - Lyon where the train service is pretty good and the station is well situated so it may not be that large an issue..