The key here is that you've got to fail quickly when you're young. And no matter how smart you are, a PhD is simply not a fast form of failure -- it takes years just to get a sense of what the research life is like, let alone know if you'll be any good at it. A PhD is something to pursue after your confident that all other, faster options are deemed undesirable.
The only exception I'd make, is if you're one of those rare people who knows that you want to be a professor, with no doubts whatsoever; in that situation, you should get into the best graduate school possible, as soon as possible, and never look back.
That said, if you're asking the rest of us what you should do, then you don't really want to be a professor.
I'm not sure I agree with your assessment about which is a faster route to failure. A startup can definitely be a fast route to failure. Working in an established corporation can be a startling experience though. Sometimes people seem to do absolutely nothing and collect a nice bonus or raise at the end of the year, etc. On the other hand, when I started grad school, it was very clear that some students were in over their heads and they were mostly gone by the first semester. Other people did take longer to realize that grad school was not for them, but the largest group of drop outs seemed to happen around the one semester mark. I think a lot of people just have their preconceived notions of grad school undone, and almost always in a negative way.
I'm assuming that the OP is smart and reasonably motivated. If so, he's not likely to be one of the fast grad-school washouts.
You're right that there are a few of those in every class, but the average smart person takes much longer to figure out that a PhD is a soul-sucking waste of youth and vitality.
The key here is that you've got to fail quickly when you're young. And no matter how smart you are, a PhD is simply not a fast form of failure -- it takes years just to get a sense of what the research life is like, let alone know if you'll be any good at it. A PhD is something to pursue after your confident that all other, faster options are deemed undesirable.
The only exception I'd make, is if you're one of those rare people who knows that you want to be a professor, with no doubts whatsoever; in that situation, you should get into the best graduate school possible, as soon as possible, and never look back.
That said, if you're asking the rest of us what you should do, then you don't really want to be a professor.