A recent check on fares to get a budget for a holiday we are planning showed a significant decrease in fares through the day, with trains departing 10:00 being expensive and late trains departing around 18:00 being cheaper. This seemed pretty consistant across a range of dates. Obviously if your destination is close to a Eurostar endpoint you can take advantage of the lower fares, if you have a significant onward journey then your flexibility is limited.
There are still some bargains to be had, and some good value standard premier upgrades, but they are fewer than a couple of years ago.
If your onward travel involves SNCF then it is worth looking at SNCF for the whole trip including the Eurostar leg. Not sure how it all works but it seemed like we would save a few pounds on a Eurostar standard premier ticket + SNCF 1st over booking separately.
For the Disneyland crowd a change at Lille will probably still be preferable to Gare du Nord followed by RER A, particularly if travelling with children and/or significant luggage. Outbound it tends to be a same platform change, returning you have the security/border checks hassle, but its far less busy than Gare du Nord (Just ignore the particularly grumpy border staff at Lille Europe).
If you are booking through SNCF some of their offerings involve Lille Europe > Lille Flanders, bear in mind its a walk, probably 10 mins or so, similar to Euston to Kings Cross.
As far as I know Eurostar dont get any subsidies, and have to remain profitable. They also have a duty to shareholders to maximise those profits, and to be honest they have a captive market, yes there is competition from other modes of transport, aircraft/flying being the main one, but flying can be a pretty miserable experience, and if like us there are no international airports closer than a 90 min drive I cant justify flying to the Mrs (who absolutely hates it) with the 'its quicker' reason. For us the ability to take a 10 min walk to our local station and complete the journey with simple changes along the way justifies the extra cost of Eurostar