Ray, As you say Railway Modelling is extremely popular and there is big money involved however only last year one of the biggest vendors, Hattons in Warrington, ceased trading and one of the biggest shows was cancelled last year. Craft based hobbies in general, across the board are in a slow but noticeable decline.
On a similar note I looked into selling on a couple of my old motorcycles, which were bought at a time when they were considered a sound investment. One is a 1954 triumph and one is a 1930 Royal Enfield, both in perfect condition. Most dealers now will not buy them off you, they will only consider selling them on your behalf and taking a cut, or they state they are not interested. If I was to sell them I would get a good bit less than I paid for them so we are seeing similar effects in other engineering based hobbies.
As for magazines we used to have two newsagents in my local village, both as a part of a general store. One has changed hands and dramatically reduced the newsagent section and the other closed completely. So now it is not possible to buy many hobby based magazines in our village.