Yes, Singapore is mostly "cards", but not everywhere. Also, some services you'll use (like taxis), while taking credit cards, are charging extra for it. You can charge an ezlink card and pay with that. But Hawker centers and a few other smaller stores might prefer (or only) cash nevertheless.

Get some S$. You'll make your life easier that way. And SGD is a mature currency, if you don't spend it all, highly unlikely it will lose much value, you can just keep it for next time.