The trouble with this argument is that it excludes a huge proportion of the best developers out there. A lot of people got into programming because they enjoyed it but after doing it all day at work they want to do something else with their personal time. And the more experienced someone is with programming the older they are likely to be and the more likely it is that they will have other interests and commitments competing for that time.

Most contributions to opensource happen on company time atleast in my experience. I don't code much outside of work yet have plenty of OSS contributions because in the course of my work I discover bugs, desire new features, etc. All the best developers in these circumstances will atleast attempt to get their changes merged so they don't have to carry patches.