Chinapattern
The viewing terrace doesn't need to be covered. I used to use Nürnberg a great deal, and after checking in, if I had time, often went up to the viewing terrace, which is open to the elements, and has glass frontage, which doesn't make photography ideal but viewing / spotting is fine. For the less hardy there is a viewing available inside, where there is also a quality Movenpick cafeteria serving decent food and drink, including beers, at not ridiculous prices.
Special events are help on the terrace year round, including an ice rink in winter!
Entrance is free, and the terrace and cafeteria well patronised, with both enthusiasts and the general public, who use the place because it's a fairly cheap afternoon or evening out for the family.
So, why wouldn't it work here in UK? I guess the first thing is that the catering would be p**s poor, as it normally is, and our stupid licencing regulations would preclude sale of alcohol in areas frequented by youngsters, making it less attractive for dad. Undoubtedly the airport would insist upon charging an entrance fee which would also put off families, but still the airport would benefit from the rake offs from parking and catering.
I honestly think that UK airport management lives in a UK bubble, and never looks at what happens elsewhere in Europe - a problem which I have to say is pretty widespread among our population and I don't suppose for a moment that Kehoe is about to open his eyes, broaden his horizons, and think outside of the "UK box".