The problem with these intermediate stops in general, but particularly Ashford, is that every seat you sell from them is one less seat you can sell from London. So you have to make your Ebbsfleet-Paris price identical to your London-Paris price otherwise you'll just reduce your yields. It makes it very hard to stimulate demand using pricing.
It has to be the same fare. The only way it could work is some type of last minute thing - like theatre lotteries - to sell off unsold capacity. Or if there is some flighting - eg. an Amsterdam which is first stop Brussels, and then a stopper behind which does Kent / Calais / Lille... and is a little cheaper.
That was the problem with Ebbsfleet. Its users almost wholly arrived by car. It encouraged UK passengers to join the Eurostar by car instead of by train to St Pancras. That really was the only reason for Eurostar calling at Ebbsfleet. It is of no benefit to anyone who travels by train to join Eurostar. I can see the point of international trains calling at Ashford but not Ebbsfleet or Stratford.
Business travellers keep this afloat. And most arrive by car service of some type at 6am or so, for a same day meeting. Equally, those leaving in the evening services (eg Parisians returning) are probably also in a cab. People don't tend to rely on public transport at 5am for this, or for super early flights either. Those others are leisure travellers and yes, more likely to arrive by train/tube.