When I say GTA 5 I mean San Andreas, even though they're not numbering the games now it is still the 5th official installment (minus that London game which was essentially an expansion pack). Saying a game can't be that different from another because it uses the same game engine doesn't hold much to it. Many sequels use the same game engine and are vastly different. Entirely separate franchise are created using the same engines as others and both have succeeded. They have done a major overhaul to the engine, please read the Game Informer 10 page feature on GTA:SA if you're going to state viewpoints about the game since really nothing was none before that except 3 screenshots. And the other recent articles about it online barely touch the surface. And I can assure you I am not a GTA fanboy, I just happen to read every single thing in every mag I get.
As for GT4, knowing alot about the creator Kazunori Yamauchi, he lives and breathes cars and he would never dream of taking away the car licenses to include damage (something that doesn't happen often in Gran Touring racing). His testament to authenticity regarding car models and their history will be evident in what he always wanted to include in Gran Turismo games, the new 'History Feature' in GT4. He's even gone so far as to include real cars as far back as 1886. He has said numerous times that car damage is something they're looking into. Obviously he wants to include it, is negotiating with the companies to, and has said he really hopes they will be able to include it in GT5.
Furthermore it is not a Sony/Sega/Nintendo thing at all. Polyphony Digital is completely separate from Sony and is really a 2nd party. Sega actually has sacrificed car damage for official vehicle designs on numerous occasions (their arcade games). Gran Turismo is not targetted towards casual gamers, the game is very complex, the reason so many casual gamers buy it is because it's a AAA title. If you compare the sales of GT1 to GT3 this reflects that. The majority of all the other racing games on the market are more targetted to casual gamers than GT is except the diehard simulations. Just because GT doesn't appeal to you Ian it doesn't mean its s****y like Enter The Matrix. GT4 is changed from it's predessor in a helluva lot of ways, the addition of online play alone is worth the new game. There is so much new stuff it's mind boggling how they actually fit it onto 1 dvd.