Prologues have generally been playable technical previews of upcoming technology - as cool and relevant this is for the PS4 era of GT, there's been some serious issues that went on during the PS3 era. There was a lot of cool and exciting new things - damage, the ability to roll over, interiors, larger grids, weather and time cycles, full HD, course creation, the list goes on - but the games were plagued with bugs and optimization issues, making it seem like PD never quite got their games fully out of a showcase/experimental/testing/etc. stage. There was a lot of new tech, but it never seemed like there was enough content to utilize all of it (under or un-used cars and tracks, short careers, lack of customizability in arcade/quick race modes, one-time/infrequent use of features and like drifting, endurance, tire/fuel wear, off-road racing, time rally, I can go on and on).
As cool as it would be to get a taste of what PD can do on the new hardware cap, I think it's more important for them to prove that they can still actually make a solid, complete, satisfactory, fully-functional game that's replayable for multiple years, and they really need to skip the prologue phase to do that.