There are two defences at risk:Well I'm not an developer so I can't speak on "weaken defenses".
If you have technical knowledge of how the defenses would be weakened would you care to share them?
1. Apps uploaded to the App Store are inspected and certain behaviour will see the app blocked. Some of these rules are for security - for example downloading code from a remote server and executing it is not allowed (even if the app developer is trusted, this can be used by third party criminals to compromise the device. While others are to protect privacy - accessing the GPS location in a mapping app is allowed - but try that in a Tetris game and Apple will refuse to distribute the app.
2. If a developer bypasses these restrictions (only some can be bypassed), Apple can revoke the certificate and it will no-longer execute on any device with it installed. To date, they have never used this kill switch. But it is available and will be used if serious malware ever gets into the App Store.
Both of these protections, which aren't perfect but have proven adequate so far, would not be allowed if this bill passes.