To answer PulpFactions' questions:
Magazines that address "how to" subjects will probably always have a changing base due to the reader's interests. Once you've progressed beyond the magazine's normal article subjects, why subscribe. BTDT kicks in.
With the wide selection of magazines out there I don't know of any niche not covered that isn't much more available on the internet. Tractors and farm equipment is an example. With a resource like
www.tractorbynet.com why would you think about starting a magazine with that subject. There are specialty magazine that focus on collector and antique equipment.
Likewise Acres USA and Permaculture Activist are there for those who really want to work with nature and maximize their results. They are definitely not for the casual reader. Especially for Acres USA, the average person may find some of the articles challenging to comprehend.
If you want to make a living at farming then Small Farm Today, Grassland Farmer and the more commercial farming magazines like Progressive Farmer are available.
Specialty magazines exist for livestock as well.
Some magazines began during a period when there was interest in them. Mother Earth News at the beginning was much different than today. I suspect at some time it almost folded. Its most recent incarnation takes it somewhat back to its roots. When it was sold the firs time, some of the original staffers started Backhome magazine which is still around today.
Backwoods Home addressed the need for self sufficiency information when it arrived in the 80's.
Going farther into self sufficiency with primitive arts you have Backwoodsman magazine.
Best of all we have Homesteadingtoday and for Brits and others
www.acountrylife.com
I find it hard to come up with anything that doesn't have a source for information.