Keep them. If and when you run out of the in date flares, you'll thank yourself for having them if needed. I'd rather have them and not need them, than need them and not have them. That's just me though. I carry a full emt first aid kit when I'm shore fishing from the beach. I kinda go overboard on preparing for the worst in whatever I do.
I would keep them. If you're floating 50 miles out and you see a boat cruising by would you like 5 opportunities to signal them or 10?
I don't know about destabilization, I guess it's possible but they aren't going to go from "guaranteed to work" to "nitroglycerin on a stick" over night. Safety tools have so much room for error I'd be comfortable keeping them for a quarter to half of their service life (42 months) past expiration before getting worried. So at least a year and a half, but that's a complete WAG so take it for what it's worth.
Wasn't there something posted a couple years ago about someone having new flares but also some old ones and getting cited by the Marine Patrol? I think if I kept old ones and got boarded, I would not have them in obvious view.