If you make a special configuration of life, you should be ready and willing to deal with the consequence of selling that life. E.g. if I buy a purebred puppy from a breeder, does that breeder get to tell me not to breed my dog? No, they don't. And lo and behold, as if by magic, there is still financial incentive in selling specially bred dogs!

Monsanto is just being greedy here, and the net impact on their profits will be minimal even if they lose. There will always be a market for them to sell engineered life "factory fresh" even if people use secondary generation of seed.