If a game doesn't have a story line that can get stale, as racing games don't, then as long as it's building blocks are solid, ie the game's engine, then updating it via DLC as they have done with GTS, should be sufficient enough to run as a single title throughout the consoles life-span. As long as having it as a PS4 title as well doesn't hamper it's evolution.
But then that's potentially the biggest issue. I would imagine that PD won't just suddenly stop updating the PS4 version. If that's the case then you are hindering GT7's evolution potential as far as implementing fresh features is concerned if GT7 runs until the next, PS6, generation of console comes along, and probably beyond that, because you are essentially tying it to the limits of what the PS4 can achieve. GT7's core will have to be pretty robust under the hood to survive what could be another 5 or 6 years or more.