No ORY is more crowded than CDG... by law.
Even with 3 runways (only 2 at LHR), ORY is authorised not to go further 250 000 take-offs and landings on a yearly basis, so basically only 20% of the LHR slots.
AF serves about 27 domestic destinations in ORY, mainly metropolitan ones. So that's not the same type of activity for BA at LGW. In the past AF even served LRH (La Rochelle) from ORY.
AF has a flight to TLS every 15 minutes in the morning from ORY arriving and departing, so 4 taking-off and 4 landing per hour.
So ORY is more practical for a lot people from the regions.
But all of that is not why AF opens this route. I think AF wants to remind the competitors who is the main carrier on the New-York Paris route. And keep its market share.
Lafayette- BA (Open skies): we are coming!
Lafayette- Norwegian: we are coming!
Norwegian is said to open too a NY-ORY route or has a plan for that!
And the flight will depart from the same ORY Hall 3 gates where from BA departs.
This is a "we are not giving the market to the competitors", we fight.
AF is shrinking a little bit on long-haul the next years: so it has the people (too much already) and the planes for that.
And it may prove more profitable than going to Kuala-Lumpur.
So nothing about ORY lower cost base: we fight against the competition right now and the future one.