The decision to delay gtaIV was made 4 months or so before the original release date. Any further and it's hard to make the call. R* is probably not in a position to determine whether they will hit 2025 until May-ish.
Also:
GtaVI will sell for 10+ years and there is no competition to worry about. They are not going to release the game until they're 100% happy with it. No matter what it said in the trailer.
I have no inside info. Didn't talk to anyone.