Domestic flights aren't necessary

I note from a report in this weekend’s FT that when business travellers booked on domestic flights with BA were rebooking during the Terminal 5 screw-up, 50% of them chose rail as their mode of transport. Which makes me think, first, why weren’t they going by rail in the first place? And second, why aren’t we taxing domestic flights out of existence and freeing up some of those supposedly vital Heathrow landing slots? We could even use the revenue to build a useful national rail link to the airport, like at Schiphol or Paris CDG.