It may be better to search for something that can be started small but that you can eventually scale.
Once a product is good enough for people to start signing up for and using, it often doesn't require much more development work. As long as people can find it, more people will sign up than cancel in any given month.
"Passive" is a really good way to describe the experience of owning one of these little businesses, once you've dialed things in so that the support workload is measured in hours/month and you're firing up the IDE maybe twice in a heavy month.
Just curious: what if a new competitor moves in? Or is does that not happen often?