I agree that adding Stratford and Ebbsfleet would be useful, but every time you add a stop you need security and passport control, and this adds to costs. Lille already has everything in daily use, we changed there returning from a trip to France a couple of weeks ago, and on a Saturday late morning it wasnt busy, certainly compared with Gare du Nord or St Pancras. Ashford has the infrastructure, just needs manning again. I think in order to undercut Eurostar enough to make it worth the hassle to passengers you would need to keep other costs to a minimum, and adding 3 more lots of security and passport control to the 2 you need for a shuttle would increase costs and therefore fares.
Calais always feels like a bit of a backwater anyway, which is only important because its the French side of the shortest channel crossing. Onward rail connections from Frethun are not that great.
I actually think a low cost operator could make money on an Ashford Lille no frills shuttle, as Ashford had HS1 services into St Pancras, calling at Stratford and Ebbsfleet, as well as local services, and Lille Europe has good long distance connections, as well as Lille Flanders offering more local connections, which is an easy 10 minute walk.
The thing with Eurostar, they can pretty much fill their trains at the fares they are charging, and unlike our internal rail network they are not in receipt of any subsidies as far I am aware. So as a business they have a duty to shareholders to maximise profits by legal means, and that means setting tickets prices to ideally just fill their trains. Too high and they will carry fresh air, too low and they will be leaving cash on the table. When I was booking our holiday I found that some flexibility in travel dates and times can make big differences, and the whole thing becomes an exercise in an equation with loads of different variables. For example is it worth staying in or near London overnight to leave on an earlier (and cheaper) departure.
In the end we did what most people probably do unless their budget is very constrained and booked the most convenient journey, but shifting travel days by one day to save some money. If you are very budget sensitive you probably wont be looking at rail anyway. Flixbus offer single fares starting at £26. Flying should be quicker and cheaper if you believe the headline fares offered by the low cost operators but the reality is somewhat different.
From a geographical point of view we are not well placed to fly. The nearest airport with any 'useful' (to us) flights is Leeds Bradford, which is a 90 minute+ drive, then parking, and a two hour before flight check in, so 4 hours later and we are no nearer our destination!!!. Then you have additional baggage charges, as living in rural area holidays are also an opportunity to stock up on wants that cannot be purchased locally. So personal experience having used both modes is flying is actually slightly slower door to door and less convenient. If you live within easy reach of a suitable airport obviously the optimum choice would be different.