PID1 also comes with powers over capabilities and (in some cases) cgroups. You can do much of that with FSCaps, etc - but to control the bounding set, you have

PID1 also comes with powers over capabilities and (in some cases) cgroups. You can do much of that with FSCaps, etc - but to control the bounding set, you have to be pid1. So - if you want init to be able to limit what processes can do, you will be pid1.