The big breweries have so much brewing knowledge in their libraries they could brew better beers than pretty much any craft brewery in the world. Especially since they have the market power to dictate how farms grow barley, what malted products are produced, etc.
I expect them to try to diversify their profits by investing in craft breweries but not make serious inroads into the craft markets. Although they could, I don't see it. The reason is their business models rely on creating marketable products that can in drank in quanty. IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIPA is not very marketable outside of limited markets. But an iced tea flavored beer is easily marketable (especially to underage drinkers). Milder session beers are more marketable so you might see some competition in those markets but I'm confident we're not going to see serious contenders for beers in most craft styles. No Bud Light with brett, avocado, dry hopped with Amarillo, star anise, thai basil and aged in sherry barrels.