The "bus factor" analogy is just that, an analogy. It shouldn't be solely taken literally, as in "what do we do if someone important becomes incapacitated".

The point of that analogy is to get people thinking about business continuity (https://en.wikipedia.org/wiki/Business_continuity_planning) and how to mitigate various types of risks. The keyword being mitigate. One cannot eliminate risk, and no sane customer expects their use of a client or software to carry 0 risk.

If a vendor tells a client that nothing can disrupt their ability to deliver their product, they are either lying, or have failed to understand the risks their company actually faces.

And the final point, "my partner can take of it". That addresses only one, frankly minor aspect of business continuity, which is "what happens if someone important becomes incapacitated". The number products that failed because someone important literally died (not just left the company) is probably extremely minute. There are way bigger risks a company should focus on mitigating first, including:

- software fails to meet regulatory compliance (if applicable)

- company fails to manage finances properly and has to close shop

- someone important leaving the company

Can some of these be mitigated by adding another partner? Depends on what they bring to the table.