The problem with any project is that once it becomes a success you need to maintain it. Often this also leads to more funding and a lot of extra features. Now to stay ahead of competitors the product needs to iterate fast and release features every day or week. Test automation lets you do this.
You need test automation to:
- let developers refactor safely which is essential to keeping the code-base clean and extendable. If the developers do not have the confidence to refactor, the code will become a mess. Manual tests are to slow to refactor in small steps, leading to big refactorings with lots of bugs and long test-dev-ping-pong-cycles.
- trust in your product enough so you can continuously deploy automatically. Releasing often is key to staying ahead in a modern world.
Third, I would say humans make mistakes and take shortcuts, certainly with manual testing as it is at the end of the process. Most things on the end of the process are optional, certainly if they are boring. Manual testing at the end of the cycle is boring. If you practise TDD or BDD and test-first automation you might prevent most human issues here as well.
As an Agile tester and Agile coach, I would always advise to automate most if not all of the testing efforts. Sure you should critique the product manually with exploratory testing for example, but any testing that is repeated should be automated.
Also read these pages about technical excellence for a project.