I never understand why there are calls to expand the awful travelling experience which is Heathrow....
Notwithstanding its location and congestion, the main issue is access. It is impossible to reach by mainline train, the 'Heathrow Express' only serves Paddington and is absurdly expensive - whereas Frankfurt, Munich and Köln have their own stations on the main DB network. It's about 20% of the cost to reach the centre of Frnakfurt by train compared to London.
The alternative, which again only serves London, is the underground system - a noisy, cramped and deafening experience compared with continental equivalents.
Access by car is no better. Once you're off the M4 or M25, it's a drive to a car park a long way from the terminals (I admit I don't know if it's as bad at T5 as I never travel ba), where the cost is pretty steep. Not too bad inbound, but when you've trudged through T1 to wait in the cold and rain for the car park bus to turn up, then crawl its way around the inner airport congestion, you know you're back in the UK.... If you follow the signposts leaving Thiefrow, you are taken along the A4 through Slough to the M4 - the route to the M25 is a well-known secret, it seems.
Gatwick is at least on mainline rail - and you don't have to go via. London. Stansted has a fast rail service, as does the increasingly busy Saarfen'.
Whereas Birmingham has excellent connections with both the main rail network and motorway system. You can either walk from the long term car park or wait for a bus - and parking is much cheaper than at Heathrow. The central airport area is compact, yet works well - although the security queues can be an utter shambles at times.
Arrival delays at Heathrow are quite common, but rare at Birmingham. So, all-in-all, from anywhere west of Oxford, Birmingham is a much better choice than the hell of Heathrow. About 13 years ago, I used to fly from Stansted to Frankfurt with buzz on a regular basis and could easily do the whole journey by public transport - it was reliable and good value. But the coach service from Oxford deteriorated and Stansted itself became busier, so when buzz folded, it was back to Lufthansa from LHR - until I discovered Birmingham. A couple of years ago, I made the mistake of travelling with Germanwings from Stansted, never again! The crowds were so bad on return that it wasn't even possible to leave the monorail platform.... Rather than risk 'ordeal by M10/M25/M40', I drove back cross-country - at least that aspect was pleasant!
However, the big guns will always press for further expansion of Heathrow - although 'Boris Island' would be the 21st Century solution. The ridiculous HS2 scheme being forced upon the UK might have a 'Heathrow spur' and will no doubt be used as an excuse NOT to proceed with Boris' scheme though.
I still think that Upper Heyford deserves further assessment though!