By the time the first plane leaves the tarmac of the 3rd runway, most/all of the noisiest airframes will have gone and there’ll probably be a significant number of hybrid/electric aeroplanes operating out of LHR.
As pointed out above, the likely location for the new runway is mostly open land that has been left that way for a long time (
look at Google maps satellite pic), for this precise reason. Compared with, say, HS2, the impact will be minimal and the benefit more material.
I don’t believe that many people moved into the immediate area of Heathrow in the last 50 years without noticing there was one of the busiest airports in the world there - in fact a large percentage will have done this because of employment caused by LHR. Also, things have come a long way since the days of Concorde, Trident, 1-11 and 737-200 in terms of absolute noise and noise footprint. I lived in West London for a while and was more disturbed by motorbikes, lorries, plus police sirens and helicopters at 2am; the jets got mostly drowned out by the ambient racket...
Naah, the whole Waterside/ R3 vulnerability has been a known issue ever since Waterside was planned..
And I expect they are looking forward to the compensation!