This is commonly said on mumsnet but its such a ridiculous point. Posters are very well aware that they will be (may be) MIL at some point and frequently say that they hope not to be overbearing or rude or excluding to their future DIL/SIL.
Very few people, that I can see, who don’t have a personality disorder to start with choose conflict with their inlaws. But its really obvious that families have a hard time negotiating absorbing a stranger, from another family/culture/village into themselves absent strong cultural rules about how that is to happen and how superior and subordinate family members are to act.
When I worked as an anthropologist in a rural, caste based, community, structured around patriarchal principles of land ownership, it was an obvious goal of marriage strategies to preferentially marry only biddable young women from the right caste into the family so that they would “know their place” and accept the family/caste rules of precedence, order, purity and pollution.
The modern mumsnet stories of MIL/DIL conflict generally resolve themselves into rather typical conflicts over precedence and service between older and younger generations where modern adult women (DIL) have education, work, and family duties which make them less likely to fuss over or defer to their husbands parents. If husband comes from divorced parents they may be expected to flatter and care for two or more parental units and negotiate conflicts between FIL/exMIL/new partners that their own DH can’t or won’t manage.
Rather than looking at this situation as a clash of good or evil personalities—though it may look that way from the inside—I tend to look at it as one of a mismatch of expectations about who makes family “fit and function “ after the first intimacy of the nuclear family.