In general for some reason cats, dogs or any other pet (Lockheed Martin - Kitty Pryde's dragon for example) has seemed to capture an audience. I remember when it was only Lockheed. Almost nobody had a pet, at least Lockheed helped to a degree.
Hawkeye got a dog, which was a part of his solo book. Why? I don't know.
Ms/Captain or whatever Carol Danvers is these days had the alien cat and still does as far as I know.
Black Widow had a stray cat that she never took into her house but took care of, though eventually she didn't. Disappointment there for me but again the book is suppose to be about her, not the cat. But the draw for me was the cat because after her killing whomever, she would go home and not be an assassin for half hour or so. Be normal and taking care of something that she didn't need to kill but actually care about, it gave her some humanity.
Re: Fanfic, it's amazing how many people get so much wrong or incorrect information from not reading source material. And how much is copied/replicated based on fanfic instead of the source material. Let's not forget wish fulfillment - (the most inspired thing for fanfic writers.)
Re: Spiral death of the comic book industry: to me is just poor writing in general (KT and TH really ruined it for me, JW did nothing to make it better). Letting good writers leave, hiring ones that are mediocre and editors that don't do their job. It's corporate now, whatever gimmick works to sell a book. Don't get me wrong, I love the flea-bags and how they became part of Gambit's background but on the other hand if the company stayed true to his character he would have taken them to a shelter for adoption knowing he couldn't take care of them due to his life style. (IMO) - Rogue makes it worse, Lorna was a better choice.
But going back to my Black Widow example, maybe it was grounding in some way, showing the character actually cared about something other than thieving (Rogue aside). FYI - all of the cats are named after Disney character cats. Corporate influence too much? I think so, but they are trying to sell their books and doing whatever takes to do so.
Know, I'll edit my post a bazillion times. LOL.