Professor's Research Progress
As the most recent news, trainers with less than 10 luckies can trade pokemons from July or August, 2016 for guaranteed lucky. I would wonder whether we will see Professor extend/release the restriction later (a while/a few months who knows) to like September 2016 or even whole 2016. I got a second Acc starting September 2016 and haven't traded any mon using it.
Answers
They will probably adjust things going forward; I can't imagine there are too many Trainers out there who have been playing regularly since Sept 2016 and don't currently have less than 10 Lucky Pokémon, so my bet is that they're going to add something to the Friendship Level bonuses like:
Good Friend: Baseline chance for Lucky
Great Friend: +5% Chance for Lucky
Ultra Friend: +20% Chance for Lucky
Best Friend +50% Chance for Lucky
Rather than making it based on the length of time you've been holding onto a Pokémon.