Earlier quoted context omitted.
Is this an issue of complexity or lack of isolation? More isolation means more complexity, but at the same time, this issue should crash the iMessage app itself, not the whole system. The fact that springboard even knows about iMessage structure is crazy...
Complexity. You don't really need isolation if the code is so simple as to obviously contain no bugs (instead of containing no obvious bugs.) Instead, we have a whole industry built upon encouraging the creation of things as complex as possible, and working around the problems caused by that by adding even more complexity, mainly because it means people have more to do. http://countercomplex.blogspot.com/2014/08/the-…
So you're kind of right, we wouldn't need isolation... if not for the fact modern mobiles exist.
But even then - people are creative. Software may be obviously bug free, hardware may be obviously bug free... then someone comes up with rowhammer and you're owned by pure physics. I'm not sure "obviously bug free" even exists.