The whole business does not have to close when one partner wants to leave. The remaining partners may purchase his share at a mutually agreed price, or another buyer may replace him.
For a large long-term project, the partners may also agree on valid conditions that protect continuity and prevent destructive immediate liquidation.
This distinction will be used repeatedly in later applications. Before calculating profit or designing an exit, first identify who owns what, who works in the business and which rule governs genuine loss.