I have mostly flown transatlantic on Delta or Icelandair and both offer free seat selection in regular coach. Once flew Condor where it is not free, did not pay for seats, and it worked out, we were all seated together.
Flying Air France on an 8 hour transatlantic 777 eastbound, a 787 westbound. I'm planning to reserve a set of duo seats on the way over (the 3-4 rows of pairs of seats at the back of a 777 along the outside) since this is an overnight flight. I paid for duo seats once on a 15 hour flight and thought that not being seated in a row of 3 was worth the money if trying to sleep.
Considering not paying on the return daytime flight and seeing what comes up for free at check in. Would prefer to not have a middle seat but don't really care about being seated with travel partners. Note that since I am connecting, I would be able to check in for the transatlantic flight more than 24 hours in advance so would get a bit of a jump on nabbing aisle/window seats, but maybe that is true for most flyers.
One point of view https://flytrippers.com/never-pay-select-seats/