Good Plans Increase the Chance of Success, They Don't Guarantee It

Even the most perfect and well-researched plan is not a guarantee of success. The business environment is too dynamic and unpredictable.

A good plan can only increase your chances of success. The job is only 10% done when the plan is complete; the other 90% is implementation.

Unforeseen Competitors: A competitor can emerge from an entirely unexpected quarter with a product that invalidates your "perfect" plan (e.g., Fairchild's 3870 emerging to challenge Intel's 8048).
Market Shifts: The market can tighten, customer needs can change, or a new technology can emerge, rendering parts of the plan obsolete.
"Unkunks": Development is plagued by "unknown unknowns"—problems you are bound to have but cannot anticipate, which cause schedule and cost overruns.

A good plan is not a rigid script to be followed blindly. It is a foundation that:

Provides a clear general direction.
Helps the company avoid catastrophic initial mistakes.
Forms the basis for intelligent adaptation and implementation in the face of unforeseen challenges.

Never trust the plan completely. Success depends on the ability to execute and adapt when the plan inevitably collides with reality.