"What prevents another group of infringers from also asking for re-exams? This process could go on forever."

Actually, not true. Another infringer would have to find another, different basis to prompt re-exam, which would have to be accepted as a valid basis for re-exam by the PTO. You think there any other "unturned stones" out there that haven't been discovered yet? A lot of people have already been looking - for quite a long time.

So, IMO, while the possibility exists, it seems a very slim possibility.

FWIW,

SGE