Most people that all ready own a game come back to at least try new content when its added, especially if it is free and not paid DLC.

Speculating on player numbers is kind of moot since every person has a unique viewpoint on what is worth it. Heck I know people that still play EA games that are abandonware now. I still have old full release software (back when that was the only way you could do it) that was put out by companies that are long gone that was great fun. If someone made an emulator for the required OS and I had a computer that had a floppy drive there are some older games that I would still be playing. If they were remastered I'd buy them again in a heart beat.