Absolutely agree with this and everything you said here. I think the only thing I see differently is that while someone should have some relevant experience before starting an ISP (or any business) is that it doesn't necessarily have to be networking. For example, some of the biggest challenges I've seen WISPs face as they try to grow are: obtaining lease agreements for wireless relay sites, hiring and training technicians in large numbers, and in logistics around keeping stuff in stock that they need to grow. Someone with experience from a different industry in some of those areas might do just as well as someone with networking experience.
EDIT TO ADD: One of things I find kind of difficult to communicate with people who are interested in it is that it's just another business, and doesn't really have any better or worse odds of succeeding beyond a few years and making money than any other business, including restaurants. People sometimes seem surprised that you can make money at this, but, of course you can! You can also make money running a taco truck or building custom furniture. But you can also lose a lot of money and time.