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.
Whether the travelators work or not is nothing to do with whether it was built yesterday or 20 years ago, it's a matter of maintenance. T4 is only 20 (?) years old and yet the travelators are always going wrong. If it's serviced regularly and refurbished at the manufacturer's recommended intervals, a travelator should be much more reliable than the ones at LHR that seem to be left to wear out on their own.