Mag, my sympathies on your loss. I've had similar thoughts about my mom and my aunt, both of whom spent seemingly happy lives as JWs. They didn't perceive it as a waste, even if others did.
I think there is some validity to the argument regarding personal responsibility for having brought people into the religion. I feel some guilt myself for having done that. But we all unintentionally do things that could potentially harm others, so that's a whole other argument.
The thought I do have regarding people like my aunt, for example, is that she sacrificed many things for her future hope, such as having children. My mom sacrificed things on behalf of her children (i.e. our education). That makes me sad, because they gave up those things for nothing.
But I think there is something admirable about experiencing life to the fullest regardless of your belief system and being able to die satisfied with how you have lived your life. Your gran, my aunt, my mom - they all died believing they were fulfilled and had been faithful. That's a good thing, IMO.