I think your question in the OP is really less about the actual dollar amount and more about a "sure" retirement. Nothing in life is "sure". Live is all about compromise, adjustment, and ultimately, acclimation. I personally don't put all faith into the 4% rule. I'm about 90% confident with "2% rule", but also optimistic that 4% or better will actually be the norm. I definitely think it's wise to diversify. So, therefore, I think real estate and possibly another small business that's mostly passive are critical parts to "retirement". And even then, keeping your mind and options open are critical. Everything truly is temporary. So, with that, I'm optimistic.
Ultimately, if you find the right work that's enjoyable, it's the only thing that makes life worth living (the ability to dream and create). I guarantee you that there's no such thing as "true" retirement. You're looking for distractions, regardless of your financial situation. The only purpose of money is to make you more of who you already are. To allow you to explore and do what you want to do. The concept of "retirement" is a modern facade that I wish this blog/forum would quit alluding to. It's the ideal that society generally accepts as the norm, without any thought as to what "retiring" actually means. In reality, the only thing it means is being able to do less of what you don't care for and more of what you do. That doesn't necessarily mean you won't ever make money again even after you've given up what you perceived to be your source of sustainability - in reality, odds are you'll come out way better.