There is no guarantee against crumbling, but you can minimize your exposure and increase your chances of success by being scrappy, as most all have done to get their community shops going. Borrowed space, donated tools and many volunteers are common strategies.
I agree that without specifics on your community, it is hard to know what the potential is. Let's look at it a different way: I've found that
established community bike shops typically have income/donations equal to about 10-15% of the larger area commercial bike shop's income. One of the cool things about community bike shops is that they are scalable, growing their client base, service and program offerings as they develop. Community bike shops typically have the community/open shop and donations for parts and bicycles as their principal income/activity, with programs (e.g.Trips for Kids) and services (Bike Valet) developing as the org grows. As service groups, labor/wages is usually the largest expense for those out of the volunteer-only phase, sometimes amounting to half of income or more.
Partnering with a local allied non-profit and talking to an attorney will increase your chances of getting off on the right foot, especially if you are new at this.