Call me a scrooge, but if they were to do that, I'd still want some sort of reimbursement. I'm not saying a full monetary reimbursement, but something significant in return for buying an expansion pack that is suddenly free for newcomers. Old game or not, there are players who have invested an excessive amount of money to own the insane amount of packs and will continue to do so.
Within that, however rests the problem in that newcomers would feel they are being punished for not having bought the packs.
The Kit idea I question for the sake of CC creators who made full kits. They still have chances of getting a little payment for their work via the creator codes. Maybe if they do not include those kits in the free release list, sure.
In the end, the only thing I'd be cool about becoming forever free for all would be Digital Deluxe thing, Kits not made by CC creators and Stuff Packs whose gameplay feature doesn't hold much merit anymore, like the buffet table and hot tubs. Tiny Living, Paranormal, Homes Chef Hustle and Crystal Creations obviously would not be included in the list of "forever free".