This is, I'm afraid, part of a vision (of which Apple is the preeminent sponsor but Microsoft definitely believes it too, they're just less aggressive) where the OS vendor is presumed to know best and the owner of the computer's wishes are occasionally considered on an "if possible" basis.

Obviously they could allow whatever signatures go into SIP to be updated, so you could make changes and then bless your changed configuration, and it wouldn't really even be hard. But they consider most computer owners to be too stupid to be trusted with that kind of power. And, they do have a point, a lot of users would happily do any super unsafe things they're told, just in exchange for a promise of a free game or a cracked version of Photoshop or whatever.

It's just sad for those of us who remember that computers used to really be almost infinitely customizable. If you didn't like something, and you cared enough, you could fix it. I assume that soon, SIP won't be a thing you could turn off even if you wanted, and while you will still be able to get a root shell on a Mac it'll eventually be on some virtualized entity whose permissions are curtailed to only a sandboxed environment.