Why You Shouldn’t Quit Your Day Job to Start a Startup
leanb2bbook.com
leanb2bbook.com
... but the title omits the "before having a good indication" part !!
I'm actually in that quandary right now (when to quit my day job and focus 100% on my startup). Right now is definitely not the time, since we don't have the "good indication" part, or even a finished MVP.
But once we do have a half-decent MVP, we'll need to focus on customer validation, which means in our case getting to actual sales meetings, and that takes a lot of time.
Also, if you're building a product (especially if it's innovative and can't easily show it before having an MVP), having a "login screen" alone won't cut it.
This is too broad a generalization. Yes, some people will not spend their time the best way. And they probably shouldn't be the ones quitting their day job. But the people this quote doesn't account for, the ones who quit a day job to focus on a product, are more likely to be successful.