I think there is something about the boomer generation. This "I'm done" approach to grand parenting seems very widespread. A lot of friends have had the same realization that you describe.
I've resolved to be the opposite kind of grandparent. I really don't have a lot more I want to do later in life than spend time with the grandchildren I will hopefully have. I like my time alone and plan to travel extensively, but picking kids up from school, spending weekends with them 1-on-1, getting lost and doing interesting things are the magic of a childhood.
As a working parent we are burdened with so much other stuff it's hard to truly enjoy the simplicity of being around a child. I plan to shrug off all my other responsibilities and just enjoy it for what it is.