The sweet spot would be place-dependent. It's 100% dependent on the locale. In the examples that are successful, we see they cater very specifically to the local travel patterns and the regional travel patterns. There is no single answer to that.
In terms of the user-pay private model, the basic answer is that in rural places, the distances and dispersed population mean that a traditional return-on-investment model is very unlikely to be profitable. If they are profitable, they're going to be profitable because they are catering to people who can afford to pay, which completely excludes people who are low income or face disabilities or other barriers to access.
As I mentioned, there's a need to consider the very real benefits that those transit systems bring but that aren't easily calculated into return on investment.