I agree with Joseph. Plan B is far from the last one you'll need.
Let's put it this way.
Plan to be wrong most of the time.
Everything you set out to do has a few components: 1. An expected outcome (this is a goal value, either monetary, traction, etc.), 2. a probability of success, and 3. the effort required to resolve the probability (achieving either success or failure, but having some answer either way).
You don't know any of these with perfect clarity in advance.
So, my approach is to understand the outcome value as well as possible (do your homework), and make the effort to test it as small as possible. Expect to repeatedly test more based on the outcome. The probability of success is difficult (not impossible to measure), but will get clearer over time, the more tests or trials your run.
This is the iterative approach (aka "lean", "agile", "set based design", etc.). Having a Plan B is not "admitting" failure, as much as preparing for the inevitability of setbacks and the need to pivot.