No implications at all. There will be still apps with PackageInstaller permissions which are verified through which then they can install 3d party apks. Also you can always use adb to install stuff on stock after this is introduced, you will just have to enable it. Im pretty sure it's about reducing malware, since as a security researcher you wont believe how many malware and scam apps I saw are being forced on Android users with simple social engineering tricks. Every week or 2 there is an article about a different malicious campaign.