The career mode in GT was tired and dated, offline career modes on the whole are not dead IMO, not if they're done right. I played GRID Autosport recently and I enjoyed the career, bouncing around the different disciplines, driving for different teams against competitive, if sometimes aggressive AI. The only downside to that one for me was forced drifting events, otherwise it was very fun.
As long as they keep the online mode as was advertising I don't see the harm in them adding more single player content because IMO, it needed it. There are plenty of people like me who have no interest in online-only/online-focused games. As it stood, GTS was unlikely to be purchased by me no matter what, because there wasn't enough to interest me.
Again, if it was just falling behind/glitches I feel like they would have given us a solid date right away. If you plan to do something by X, and you're running behind, you generally have a good idea how much longer it'll take and announce Y as the new date. They've not done that, they've gone from a set release day to a year. A 1-day window to a 365-day window. I feel like you don't put that sort of uncertainty in players unless it's because you really just don't know when it'll be ready. Look at all the Uncharted delays, they may have missed them but they gave months, they must've been relatively confident at hitting them. Yet PD seemingly went from being confident to hitting a day to not certain enough to even hit a month.