...but with passengers driving a fair distance to parkway stations rather than shorter, possibly bus, trips to their local station.
There will be hardly any 'Parkway' stations on HS2:
London Euston, Birmingham, and Manchester are all inner city locations integrated with or with short links to the classic rail network, so little change compared to changing from a local train or bus onto an inter-city train at a major station. The same goes for Leeds if the eastern branch of Phase 2 gets completed, and similar can be said for Crewe. And, admittedly away from the HS2 network itself, HS2 trains will serve existing stations at Stoke, Macclesfield, Liverpool, Preston and stations north to Glasgow on the WCML, and subsequently stations between York and Newcastle on the ECML, again if the eastern arm of Phase 2 is completed.
And I thought that the point behind Old Oak Common was that it would be integrated into the local commuter rail network in the form of Crossrail (and possibly LU?), rather than encouraging car use, though I could be mistaken.
The only 'Parkway' stations would be Birmingham Interchange and Manchester Airport, both sites that already have an infrastructure in place for the park and ride journeys that are made (Not to deny that HS2 services would probably result in an increase in such), and Toton on the eastern leg of Phase 2, which represents the only entirely 'new' parkway station site.