If model railways are in decline, then it's news to me. Sales of both magazines I work for are up. Show attendance is up. Numbers, and prices of products are up and they seem to be selling. Yes, the age profile is older, but it always has been. I don't remember many strapping 20-somethings at shows when I was younger.
As far as MB is concerned, as has been said, the Editor can only publish what she has been sent, and if the people in the hobby can't be bothered to write, then the mag will look thin. Colin makes a good point about the extra pages. IMHO, there were four pages of the workboat that showed that, but the rest is as good as ever. Yes, the content has changed from the 1960s, but then so has the hobby. MB has to evolve like any other publication.
Moving on to the model boating hobby declining - does anyone have any numbers for this, or is it just the general internet doom and gloom? Yes, there are less shows, but that is because model boaters don't put them on, (railway modellers do) and don't support the ones that exist (I still remember the Coleshill show which was full of boats on the Saturday because putting one on display got you in free, and empty on Sunday as the freeloaders went home) well. There are fewer model shops, but we have the web, and some on this forum boast about how they import everything from China, and don't see the point of shops.
MB is a "use it or lose it" situation. If the hobby is in decline, then a lot of that is down to those in it wanting someone else to do everything for them. It doesn't have to be. Other hobbies are going well, the massive Dortmund show is on at present, and that brings in serious numbers. Key Model World should do well at the end of the month. The ARTF plane market is booming and Tamiya keep rolling out RC car kits, perhaps we can attract people who build those to the hobby. Airfix keep tooling up plastic kits, and there is a thriving aftermarket to support those. Again, how do we get some of those people in our hobby. Featuring plastic kits in MB can't hurt in that respect.