A focused pilot is usually better when the problem is specific, like slow permits or procedures nobody uses, and the existing platform handles everything else well enough. It gets results quickly, carries far less risk and shows whether the new approach works before a bigger commitment.
Replacing the whole platform makes sense only when the existing system is the root of the problem across many processes and the business is ready for a long project. Even then, starting with a focused pilot in one area is often the best way to prove the new platform before committing to the full replacement.