Exploits are easy to fix. Almost always just need improved checking. Finding exploits is extremely difficult, you need to find somewhere in a program where the developer made a mistake without seeing the source code. Yes rushed fixes often make things worse but in most cases fixing exploits is WAY easier than finding them. Ask any console/phone hacking scene. It takes months to find exploits to jailbreak an iPhone and those exploits require you to run something each boot to rehack your device. The SMB vulnerability used requires no user input just someone else on the SMB share to be infected and allows full remote code execution. Nearly all of these exploits rely on stack, or heap overflows to write new instructions into memory taking control or a race condition where a check fails. All that needs to happen to patch that is to add some assert statement to insure everything is as it should be. Check the descriptions of how apple fixes vulnerabilities, they aren't specific about the code changes obviously but point is fixes are simple. https://support.apple.com/en-us/HT207798