I absolutely agree with you that, making your system simple is one of the way to support growth.

FoxtrotUniform gave a very good point: decoupling. I see decoupling as one way to modulize your application, and there are lots of other ways: put your code in a function, wrap your code in classes. You can even view multi-process, multi-threading as sort of modulization...

Said all this, I am thinking whether we can combine your thought, FoxtrotUniform's thought and mine together, and say:

To simplify the functionality of each module, and make modules have clearly and carefully defined/designed boundaries, would absolutely support future growth.