It would also destroy their market share. I don't know if a price has been announced for the Nexus Prime yet, but for the sake of argument let's assume it's slightly more than the SGS II. Most people aren't willing to pay that much for a phone, no matter how awesome it might be. In addition they'd lose the customers who came to Android because they wanted a smaller phone, or a phone with keyboard or a phone that was IP67 certified or any of the other options that the Nexus Prime doesn't offer. I'm sure Google have considered the options and are much happier with a good product used by lots of people over a great product used by few people.