Anecdotally, I have no degree, 1 internship, no significant side projects and now make $100k + significant equity.
If it matters, I was hired through a Who's Hiring post with no personal connections.
The biggest justification for it is probably the idea that you hire people based on their ability to learn rather than their current knowledge. This sort of makes sense if you consider how fast tech tends to change and believe that you'll be able to hold on to an engineer long enough for them to grow into competence. Of course, it also leaves you with the huge problem of trying to evaluate "ability to learn" through a few hours of interviews.
The other thing to consider is that hiring is necessarily an ecosystem. An entry level job at Facebook can net you something like 120k + reliable stock compensation + signing bonus, and a startup that doesn't at least try to compete with that is going to lose out big on engineering talent.
It all ends up having very little to do with "justifying" a 6-digit salary, and a lot more with scarce supply.
This is also why we've seen bootcamps and and online vocational learning (Udacity, Bloc, etc) gain a lot of traction. Paying $18k for 3 months at Hack Reactor might seem insane, but if it lands you a $100k job it very quickly becomes worth it.