[cbr]

    when all purchasable soybean seeds on the market
    end up being composed of seeds that originated
    from Monsanto's genetically modified stock

This is very unlikely to happen. People are careful to keep track of seed varieties. And as long as there are many people who don't want to grow GM soybeans then non-Monsanto seeds will even stay relatively cheap.

[Retric]

Patents also have a finite lifespan. Keep a sead for 20 years before planting an existing patents are useless.