GTA has quite wide appeal though, hence why it sells to well. It appeals just as much to the people who just use their console for FIFA and CoD as it does to do the people who play a variety of games.
Most people who buy PS4s and Xbox Ones will already own a PS3 and/or 360 and will probably have already bought GTA5 too. After paying out for the console, and some next-gen games, I don't think that many people would be keen on buying a game again that they bought and played 6 months or so earlier.
its does appeal to more than the core, but its the core that buy the largest chunk of the copies, where mums and dads are happy for their kids to go around shooting people in COD, GTA always seems to be a bigger no no in there eyes. people will have forgotten about the cost of the system as that would have been up to 6 months ago, it would be just like a release of any other game but its GTA
GTA 4 sold about 25 mil over 3 platforms over 4 years, so nice lifetime sales. so given you could be looking at 20 mill next gen consoles in year 1, the maths to business practice starts to make a lot of sense that a next gen version would happen.
Also a large part of the core gamer's will sell up current gen to fund their next gen console which then shuts the door on them playing GTA 5 DLC, which going on RS time scales will be at least 6 months after the base game comes out. There is no way RS will want to miss on those sales & I'm sure they saw a boost in copies sold at the time of each DLC for GTA 4.
Nobody likes a double dip, but look at how many blu rays people have that they also have on dvd, the public will do it and RS know it, a great game will always sell again on a new gen, you only have to look at the amount of double dips with HD remakes that go on to see there is a market for something like this.
Of course some gamers will still have there old system and will get the DLC on that system, but a large chunk will not have that old system and the double dip is aimed at them, if only a 25% of all next gen consoles from year 1 buy the double dip that's an extra 5 mill copies which is a 5th of the total life time sales of GTA 4