Larry, it is very hard to know what is attainable or not, that is the catch, isn’t it? A great example, imagine everyone advice to Donald Trump, 15 months ago! Imagine, advice to Elon Musk about building a car manufacturing in California! Same for Airbnb and Uber! At some point in time, most people taught pursuing these ideas are sheer stupidity until someone proves them wrong! Then, everyone says, that was obvious!
I like Mark Twain quote: “It ain’t what you don’t know that gets you into trouble. It’s what you know for sure that just ain’t so”