I read that PD will add more events to the campaign mode through updates, which will add longevity to the offline aspect of the game. And I looked at the trophy list, most of them are offline but I have a feeling that the F1 trophies for matching Schumacher's 91 wins, Barrichello's 322 races and Senna's 65 pole positions are not. They'll definitely be tricky ones to obtain if that's the case.
As for PC2, I'll probably buy it once I'm bored with GT Sport (hopefully with bug fixes and a discounted price)