> First, there is your biology. You might simply not be intelligent enough to become a great engineer.
Biology is overrated. From "You and Your Research"[1] by R.W. Hamming:
"You observe that most great scientists have tremendous drive. I worked for ten years with John Tukey at Bell Labs. He had tremendous drive. One day about three or four years after I joined, I discovered that John Tukey was slightly younger than I was. John was a genius and I clearly was not. Well I went storming into Bode's office and said, 'How can anybody my age know as much as John Tukey does?' He leaned back in his chair, put his hands behind his head, grinned slightly, and said, 'You would be surprised Hamming, how much you would know if you worked as hard as he did that many years.' I simply slunk out of the office!"
Other anecdotes lead me to believe the primary difference between those that are and those that might have been is hard work.
> You might not have the health and the endurance to work on demanding high-end projects long enough to get really good at those kinds of projects.
Bad health is a rough deal for anyone. But it is possible to succeed in the face of it. Look at Goddard[2]. There's a good argument that he didn't reach his full potential because of his health, but he still did some truly great rocket science.
> Your other obligations, like raising children, can get in the way
Rather than get in the way, I think they are often essential. I imagine it must be hard to do great things if you're personal life isn't in good shape.
There are real obstacles to "greatness", I think. Things like access to education and financial security. But I find it hard to believe that biology, health, and domestic life are enough to stop a dedicated person from performing great feats.
[1]: http://www.cs.virginia.edu/~robins/YouAndYourResearch.html
[2]: https://en.wikipedia.org/wiki/Robert_H._Goddard