Windows Defender is and always was considered a last resort, made in response to the fact that enough people don't buy AV software, and/or don't update it. Then Windows computers bomb out from viruses, they blame Microsoft, and it becomes a viscous cycle.
As I said though its a last resort, you very much should consider a real antivirus suite. Bitdefender is one of the best and only $29 for the year. If you ever get a virus that Defender can't stop you will wish you spent that money. All it takes is 1 mis typed url, or mistaken click on an email or link even in google.