I'm pretty sure that if Heathrow were to be built again tomorrow, it would be a world-class airport (well, at least one where the travellators work and the queues for security aren't quite so crippling), and will be interested to see what T5's like when I first use it.
That's exactly what is planned in the coming decades.
T5, then refurbed Central Area (bye, bye T2), new commuter runway, refurbed T4 and in about 20 years maybe even a more sensible train system whcih integrates the rest of the network without having to go to Paddington.