If HS2▸ is finished, then Birmingham airport will better rail connections than any other airport, and will be quicker to get to. After all, Birmingham airport will be 38 minutes from Euston.
So why not add two runways here and make it the hub? As a bonus, M5/M6/M1/M42 are all nearby with local and national rail lines and HS2 in a few years.
I completely agree than Birmingham Airport will do very well out of HS2 (it will be closer to London than Stansted) I am not sure it is a good location for a national hub though because it is rather close to Birmingham's suburbs and in many ways would cause as many problems for Brum as LHR does for London.
To avoid city centre over-flights an ideally sited airport needs to be North or South of the city it serves not West (as with Heathrow) or East (as with Birmingham)