All games compete with each other, if you released 7 games in a week and they were all different genres its ridiculous to say they are not competing with each other. People only have a limited amount of cash for video games and can't buy everything. Your thinking about this wrong, I'm not saying releasing GT6 on PS4 would mean they would make more money, GT games are Sony's biggest system sellers and given the choice to put it on a system that's already out and make some cash (which won't even be much if things keep going like this) or putting it on their next gen console and investing in that console that will be around for many more years the answer is obvious. Actually if things keep going like this and Sony doesn't advertise the game I think it would sell more on PS4 than on PS3, if it was on PS4 you can bet Sony would be advertising the crap out of it, but since its on PS3 I'm guessing Sony is focusing all their advertising resources to the PS4 this holiday and not gonna bother with the PS3.
1.) If I were to release a family movie and an R rated movie at the same time, do you think that these movies will take each other's sales? No. They're two different shows that appeal to two different markets. GT is a racing franchise with a very established fan base. Do you think these fans won't buy the game? Halo is a shooter franchise with a very established fan base too. Do you think these fans won't buy the game either? Do you think that the people who own a Playstation console will not buy GT just because of a completely different game on a completely different console? The answer to all these questions is NO.
2.) How many PS4s do you think will be sold by next year? Will it be more than 80 million? Of course not. The PS3 is a much bigger market than the PS4 will ever be in a year or even three. No matter what you do, you can't beat the user base. It's like putting up a Mcdonald's in New York or in Latrobe (PA, population of less than 10k). A bigger market will always yield bigger sales.