But ain't the important rating the Fox channel rating and not the FS1 for Smackdown? Why would Fox be upset at WWE for decent ratings on their subpar channel?
They are still doing damn near great on Fox.
The only real thing that should ever matter if they were going head to head, and AEW continuously beat SD right?
As long as Fox isn't looking sideways at Smackdown's metrics, WWE will be fine. Despite its ratings slip, RAW's total viewership still does pretty good numbers relative to the rest of the shows on USA. There's a little more friction with Fox (see their confusion regarding the CM Punk situation and WWE not taking the layup they gave them with Backstage), but seemingly nothing major on that front. We probably won't know anything truly substantive about that relationship until 2023 or so. Same can be said for AEW and WarnerMedia: given Rampage's timeslot, I doubt they're TOO concerned with its numbers right now and Dynamite is a huge hit for them.
Wrestling fans tend to blow it out of proportion because companies, wrestlers, commentators, etc. see it as a major issue. I've explained my reasons for keeping up with ratings in another post in the thread. And, honestly, it can be fun to follow if you don't take it too seriously (which too many fans can't manage to do).
As for demos, I think someone else explained it in the thread, but 18-49 (and particularly 18-34) is seen as the group with the most disposable income and the most fluid brand loyalty due to the average lifestyle of someone in that demographic. Younger than that and you're really in the process of developing your tastes with no disposable income, older than that and you're out of touch with a family and mostly set in your ways. You get the 18-49s, and you've made a captive audience out of an audience it can be hard to do that with for the reasons stated above, among others. Of course, if you're a channel like Fox News (the best shows of which consistently beat every show in their time slots in total viewership), then you're not going to really look for that demo, since they're not that interested in what you want to say to begin with (naturally, they kill in 50+ demos).