I posted too soon.
I can understand her disappointment if she believed they finally had a solution to all their problems being handed to her on a plate, and now it's all fallen through.
But when that happens you can't lash out like this and start saying things like "we gave them their only grandchildren" or "they haven't worked in ten years...my mum still works and contributes to society."
You just can't. PILs have obviously worked up to retirement and own their house outright. They might not be working, but they are still contributing to society. They have paid in all their working lives, they are saving for their grandchildren's futures, they are still buying things and paying for things, and who knows what they do as retirees that may help other people.
You also can't say "we gave them grandchildren" like you did it all for them and solely for their benefit. We had a child because we wanted one, not because our parents wanted a grandchild. I'm sure the writer of this letter was the same. She had her children because she wanted them, nothing to do with giving anybody a grandchild.
So she's had her disappointment. She has to now start to accept it, and consider all the hundreds of reasons why it was probably a bad idea anyway. And accept that her PILs don't have to just hand over their home because it suits her. And look at other ways to get the house she wants in the area she wants, that doesn't include turfing out her PILs and expecting them to be happy about it.
The bit I wondered about most was that her FIL doesn't like confrontation. That could have been a big clue to how this arrangement was made in the first place. If it wasn't PILs idea, they might have agreed just to keep the peace and spent weeks feeling scared about how to go back on it without a big fight with a very angry woman.