Rampant proliferation of "new" also breaks the hiring process. Companies (i.e. "in-demand startups") want new stacks, and they want to hire people who are experts in everything that didn't exist 2-3 years ago (literally, 5 years experience with a 2 year old platform).
When that's the case, you're either a high school or college kid who can spend all their time learning new things, or you are old and learn new things a few hours a week, maybe nights and weekends.
You can't compete with someone learning (and iterating!) 40-60 hours a week when you yourself can't throw away all your 40-50 hour per week obligations and responsibilities just to keep up with them.
I'm going to go out on a limb and say the bigger problem with 40-50 hour per week obligations is that it doesn't leave you free to work 80 hour weeks.
A lot of startups really aren't doing anything that technically exceptional, but if you want someone to spend their evenings fixing bugs in your bog-standard Rails app, it's much easier to persuade the guy that spends them fixing bugs in bleeding edge javascript frameworks than the guy that spends them with his wife and kids.
When that's the case, you're either a high school or college kid who can spend all their time learning new things, or you are old and learn new things a few hours a week, maybe nights and weekends.
You can't compete with someone learning (and iterating!) 40-60 hours a week when you yourself can't throw away all your 40-50 hour per week obligations and responsibilities just to keep up with them.