This guy once gave at talk at my university where he said he regretted not doing a lot of things because he was working and now he was too old to do them because of his health.
He sounds like someone who would give a great talk. I can see that happening for sure. I've devoted my life to a lot less before realizing there was more out there. These people put so much time into their work there's little left for a broad life - unless you make some clear boundaries like Mark is doing now. Maybe this will be a one time thing and, as you suggest, he may regret not making more time for other areas of his life. Maybe he'll manage to carve out more time for himself.
I don't buy regret. From what I've read so far, people who worked too much regret not doing things with family, etc. and people who lived bog-standard GenPop family life regret that their life had no meaning. So I guess death is just one big paradox of choice.
https://en.wikipedia.org/wiki/Seymour_Schulich