That's a tempting argument against expansion (others are available
), but it's actually a bit of a red herring.
I've never quite understood the "ban transfer traffic" preoccupation of campaigners.
Apart from the fact that it's a tad impractical to frog-march every arriving passenger to the Tube to make sure they don't board another departing flight, many routes from LHR are only viable by virtue of carrying transfer traffic.