They can't just stop or they'll face bankruptcy by paying Apple damages. No doubt Apple made them sign an ironclad contract...and most likely a few years notice is part of it. Apple isn't stupid.
Not to mention that Samsung isn't doing Apple any favors, they're making a fortune because of the Apple contract. If they decide not to renew it, that's their prerogative but I'm sure Apple has plan A and B ready. Someone else will gladly take their money