The thing is that Eurostar themselves are a massive enabler for that, why would they open check-in 2 hours before departure (they do that in Paris)?
If you don't want to deal with queues at GdN, have someone posted at the stairs/lifts on ground level and make non-BP passengers disperse on ground level before the hours mark.
There is a lot more room in Paris, albeit a lot of people never walk down to the end where there's always loads of room!
Admittedly, there are times where there's no real attempt to limit entry at St Pancras - but when there are staff there, they do have separate lanes for each train and can turn people away.