I have also thought about this a lot…I lost my “husband” (common law, monogamous, my first and to date only love, best friend, father of our 2 children) of 23 years 2 and a half years ago. Yeah, I miss him. A lot 
His going nearly killed me…seriously…it was HARSH. I still deal with it but it is much better now. For a long while, I was just lost and couldn’t really find much of a reason to live (aside from our kids, and that responsibility, at the time, brought me no joy, it just burdened me with fear and guilt because I felt as if I wanted to die but felt if I did, I would be doing them a horrible injury):o
As I said, I feel better now, most of the time, and take so much joy in our kids (18 and 11). I just wish their dad could be here to do the same.
I think this experience has really desensitized me in one way and RE-sensitized me in another.
Meaning, I realize there are really only a few people whose death would strongly affect me (my 2 kids or myself). It is not that I don’t care about the fate of others, but I guess this hit home so hard that I was forced to face reality and come to terms with what really matter to me (it is ME and MY KIDS).
Also, I have been through almost the worst loss I can imagine (next to losing one of my kids) and so the fact that my MIL and FIL and even my own mother are going to die soon really is not something that bothers me. I am a bit emotionally dissconnected and simply see it for what it is and know it will not affect me in any serious way.
I think it has to do with spending the last few years in emotional overload…maybe my circuits are just fried. 
And it has to do with the simple fact that these people (yes, even my own mother) have simply not been in my life to the same degree as my late spouse or my kids over the last 20 yrs.
Yes, I LOVE my mother (whom I have been dealing with a lot lately due to a recent injury and her needs as an elder who refuses to accept assisted living and me being the only one available to help out) but I simply do not feel the degree of attachment to her and her impending death that I did/do for my DH and my kids.
Same for my in-laws. I go through the motions, I love them, but I know their passing will not seriously affect me. I love them, I will miss and always remember them, but I will not mourn as I did for my DH. I will not pine away or wake up weeping for many months.
So yeah, kinda sad, but only 2-3 has to be my answer at this point.