Money isn’t the problem; you are.

If you have the skills, interest-free money is available for you to use.

Business is a test of capital—more precisely, of the cost of capital. Many businesses can be operated by many people; the real differentiator is whether you can secure funds at a lower interest rate. If your loan interest rate is low and your credit limit is high, your advantage becomes clearly apparent, while your competitors fall behind.

What does a stable yield depend on? First, it should not depend on specific business deals. Second, it should not depend on specific people.

When your friend agrees to pay you 50% interest at year-end, there are prerequisites—namely, that his company is still in operation and that you are still alive. His company continuing to operate means it depends on a "thing" (a business), while you staying alive means it depends on a person.

If his company no longer exists, he has no capacity to pay. What could he possibly use to pay you?

If his company still exists but you don’t—for instance, if something happens to you—your wife and children might come after him to collect the debt. Would he honor that?

Whether to prepay your loan boils down to a single question: Can the stable yield you earn on the money you could pay off—but choose not to—outperform your loan interest rate?