I think a lot of people would argue that AI wasn't available in GT4 either
but seriously:
I'm in agreement with others here. I, being one of the relative few who does play GT4 online with xlink, can say that while it is fun, the online play should not be the primary focus of the game's development. What keeps me coming back to the game is trying to perfect car setups, understand the physics model (another area that could use improvement), and to find close racing in GT mode - not to xlink.
The online thing is fun, and I suppose online racing leagues would be enjoyable for some. But I like the feeling of accomplishment I get for progressing through the offline GT mode. Improve the AI for closer more realistic racing and it only makes sense that this portion of the game will become more enjoyable and immersive.
The GT series touts itself as 'the real driving simulator.' In that case it should aim to offer an authentic racing experience to
everyone - not just to those that are able to play it online.