I do think op has a point.
Because while there are clearly some toxic parents out there, there seems to be an expectation that the MIL will be a toxic resentful woman who is jealous of the time that her DIL spends with her precious son. And thus, if you believe in something enough, you can imagine it happening, iyswim.
Over he years I have seen posts on here along the lines of: "my mil is a toxic woman, she gave my child sweets/spoils them/bought them toys I don't approve of" and so the list goes on. It's almost like competitive mil bashing but without anything to compete with, iyswim.
I am also often shocked at the comments from posters along the lines of "you are your dh's family now, not your MIL." does that mean that those of us who have boys will cease to be related to them when they grow up? If someone posted on here that their dh had told them that he was her family now not her mother, there would be outcry, and he would be accused of being a controlling arsehole, but for some reason it's acceptable to say so the other way around?
And maybe, just maybe, if a MIL feels that her DIL is resentful of her/expecting her to be a toxic woman, she's less likely to make the effort to form that relationship.
I think that it's a two-way street tbh, and while there may be some horrid MILs out there, there are certainly quite a few toxic DILs as well, except it seems to be more acceptable for the DIL to be a bitch to her MIL, as purely being a DIL qualifies her to do so.
Being the mother of a son it scares me actually, that he might end up with someone like some of the posters on here, who will resent any involvement I might want with my son/grandchildren.