I think there's respect and there's respect.
There's "respect" as in "simple decency". Namely, that you treat people with courtesy and good manners. That you owe to everyone. Adults should show this type of respect even to children. It's not reasonable to expect that your children will have good manners and show courtesy if you don't model that behaviour for them.
Then there's "respect" as in "respect for authority". That has to be earned.
Just because Mum raised x number of children, and they turned out okay, doesn't mean that Mum is an expert. (In fact it is quite possible that the children turned out okay IN SPITE of Mum, rather than because of her, in some situations!) Moreover, Mum isn't an expert on THIS new baby. As we learn better, we know more. CPR guidelines have changed since I first became a nurse, for example. Does that mean we didn't save anyone with the old guidelines? Of course we did save people - but now that we know more, we do better and can save more, with better outcomes.
I think what it is is that people are willing to put up with a lot less than they were in the past. Sadly, I have heard people say, "Well, MY MIL was an absolute cow but I tolerated her for the sake of the children." Well, good for you. But that doesn't mean that you get the right to be an absolute cow to ME and expect ME to put up with it.