Rebooting the system if init dies is a hack
I feel like I should say this explicitly: rebooting the system if init dies is a hack. It's the easy thing to do but not the right thing. V7 Unix more or less ignored the possibility of init failing; when BSD started considering this situation they took the easy way out of 'handling' it by just rebooting the system. Everyone since then has copied BSD (probably partly out of compatibility, since 'everyone knows' that if init dies the system reboots, and partly because it's the easy way).
You can argue that if init dies something terrible is going on (especially after the kernel has armored init so that you have to work very hard to terminate it) and this is generally true. But rebooting the system is the lazy way out, especially when this is determined by the kernel instead of user level. It might certainly be sensible to configure your system to immediately start a reboot if init ever dies and is restarted by the kernel, but at that point it's something you control at user level; you might instead ring lots of alarms and see if the system could limp on. And so on. From some perspectives, 'reboot the system if init dies' is the kernel meddling in policy that should be left to other levels.
The right thing is to provide some way to recover from this situation. I outlined two plausible approaches yesterday; there are probably more. Of course this is more work to design and program than just rebooting the machine, but that's common when you do the right thing instead of the easy thing.
It's kind of sad that almost everyone since BSD has simply followed or copied the BSD quick hack approach (even the people who reimplement things from scratch, like Linux) but this is pretty typical for Unix. If some Unix did try to do it differently I suspect that there would be people complaining that that Unix was over-complicating init.