The 10-Year Rule
When you build, you're not committing to a project. You're committing to a product — with 10 years of maintenance, upgrades, security patches, and staffing.
Build when: it's your core differentiation, no vendor solves your specific problem, or vendor risk is unacceptable. Buy when: it's commodity functionality, time-to-market matters more than customization, or the vendor's R&D dwarfs yours.
The hidden factor: buying creates vendor dependency debt. Building creates maintenance debt. Choose your debt wisely.