Although point-to-point from the centre of each city, the train is competitive, if you are connecting at LHR it is far more convenient to fly from NCL. That's especially true if you have luggage. Every additional connection - either Piccadilly from King's Cross, or Circle then Heathrow Express - adds waiting/connection times plus the additional uncertainty of delays compounds.
HS2 itself will only benefit Newcastle substantively once the Leeds leg opens, and as others have noted there isn't a huge time saving for London itself - but along with Crossrail it will make taking the train from NCL to LHR much more convenient (via Old Oak Common). Personally I wish they'd pursued the idea of a HS2 spur to LHR (and onward to e.g. LGW and Ashford), and through-ticketing for airlines. I guess it remains to be seen whether BA (or whoever) feel that there remains sufficient connecting traffic to continue running a regular service once HS2 Phase 2 opens.