I do have compassion. Where I have a problem is with people presuming to know the minds of their AC.
I think some of this might stem from mismatched expectations. Back in the day, yes, people spent more time with extended family. BUT. Mums generally didn't work. Children had fewer after-school activities. Evenings were family time. Few families divorced. There WAS more time available for extended family.
Today, many families require both parents working to make ends meet. Children are involved in more activities. Many marriages end in divorce. Let's say you want to see your GC once a month. If the parents are divorced, and each set of grandparents gets the child one weekend a month, each parent gets only one weekend a month with their own child. Or, there could be differences on parenting.
I don't think most DILs feel threatened by their MILs. At least, I don't, my SIL doesn't, and I don't think my sister does. I think some of it comes from mismatched expectations and if the expectations are not met, it's perceived that the children are being used as pawns. Some DILs don't want to send their children around if their MIL has been rude to them because they are concerned that MIL might talk smack about them to their children, or at the very least, it's rewarding MIL with exactly what she wants - to see the GC without that pesky DIL getting in the way. This can reduce the DIL to feeling as if she's nothing more than an incubator for MIL's grandbaby. Sometimes, too, there's perceived to be a problem because the family has a "you handle yours, I'll handle mine" policy. Not all wives want to become the "social secretary". They expect their husband to handle his family. Then the son doesn't contact his parents, but it's perceived to be the DIL's fault when in fact he's just not good at staying in contact.
I am NOT saying that any of you have done this. I'm simply pointing out that this is a common theme I have heard from EAC. Do you see why a DIL might not want her children around someone who treats her that way?