Considering my internet habits I would never even considering relying on Defender as my sole source of protection nor would I recommend to anyone that they should. Microsoft, yes "Microsoft" recommends that Defender be supplemented with a third party anti-virus product. Why would anyone not heed that recommendation? In most instances, but not all, the user is the final gatekeeper but often enough even due diligence is not enough and it is for those times I will always hedge my bet with additional protection.
Generally, most have a significant investment in PC hardware not to mention the user/owner time investment in setting up, maintaining and troubleshooting their device so why would anyone pass on additional protection? There are a number of good applications available for free and many more where the annual subscription may amount to a couple of bucks a month. My monetary and time investment are worth more than that.
And finally, no anti-virus/anti-malware is 100% foolproof and never will be but they do provide another layer of protection which is why a back-up strategy needs to be in place to recover from unforeseen events. Regular Restore Point creation is a must, Custom Refresh Images is another option and the ultimate solution is creating backup system images to an external device.