The core question is - Is career Mode in GT6 convincing?
GT4, looked at for comparison, was more immersive (for me) than 5 or 6.
I played GT5 completely offline till the end investigating its 'Career Mode' and the playability of the original game (Version 1.00) as released, and yes, there was a game in it and there was a progression and some sort of Career, as well as a career managing a racing team, but still not as captivating or as immersive as GT4, though there were areas of GT5 that improved on GT4.
In comparison to both, GT6 can be regarded as sterile. Yet, GT6 also has sectors that are unique and immersive - like the Senna events for instance. The impact that A-Spec Seasonals has on the career mode is to inject fresh blood continually into the stream, but this, while palatable, won't satisfy for too long.
A story is needed, a 'MY GT LIfe' progression that the player has to 'live' through, and has to make the right decisions and input sufficient playing skill that will make the game reward and level-up the player towards a final objective. For now it's a smorgasbord of events that doesn't come together sensibly as a whole.
Something interesting i found out recently - when we play the Seasonals, we are actually playing 'offline' (as in hosting the event on our own consoles.)
You can pull the internet plug and still continue to play an A-Spec event (after you've clicked on it) many times over.
In essence - for that time - it's part of your offline 'career' mode.