Today’s issues regarding Heathrow expansion have arisen due to concern over emissions. It makes absolute sense to do everything possible to manage air travel growth through modal shift of domestic traffic from aviation to rail. HS2 at OOC does not deliver that, whereas HS2 at Heathrow along with protected connections and baggage transfer would. Those who believe the environment is a priority are delivering an illogical argument if they campaign for flight constraints and then ignore the potential for mode-shift.
Why does going via Heathrow make it more likely that people will shift from air travel?
Providing an easy too use rail service from (say) Reading to Manchester would likely attract a lot more people to rail than getting them between Heathrow and the rest of the UK.
Domestic air travel accounts for ~30 million passenger movements a year Virgin Trains accounted (on its own) for ~40 million passenger movements.
Now many of those air movements are between the mainland and islands and Northern Ireland. Therefore HS2 could easily be a game changer in terms of how people travel within the mainland (there's always going to be those who'll fly from North of the Central Belt to elsewhere just due to the distances involved) as it would put rail on a more equal footing.
Especially given that you get money back with MUCH smaller delays on the train compared to flying.
Domestic flights will likely be hit hard by HS2, as it wouldn't take much of a shift for the number of flights each day to drop, this in turn would make flying less attractive. This would then make the train more attractive, resulting in more flights being cut.
If this repeats too much then routes become unstainable to keep running (even if there's some who would rather fly).
Take Manchester as an example most people do by train as it's 2 hours, cut this to 1 hour and the number of flights will drop significantly (not that there's that many anyway).
Central Belt to London being sub 4 hours will start to erode the number of fights, even more so than what's already been happening.
I wouldn't be surprised if in the future it was found that the airline industry knew of the risk of this happening because of HS2 and so were supportive of those who were opposed to HS2. (Note I'm not suggesting that they have and certainly not saying that any individual or organisation has, rather if they have and it came to light it wouldn't be a surprise).