As someone who OP would likely describe as a stronger "gaining fetish" than a fat fetish though not to the point of the schizos who freak out when a model loses 20-30lbs or doesn't gain noticible weight for awhile, I actually agree with a lot of what OP is saying.
To me a lot of it is more fantasy than real though, online I find myself watching people Hayley and Twinkiebelle who are objectively ugly but have packed on an obscene amount of weight and reached whale status very quickly more often than more attractive women who have been around the same weight for years. In real life I'd never want to fuck someone like Hayley however.
A lot of these women clearly also have either one of or a combination of a crippling food addiction, mental issues, or an addiction to the money/attention of being a fetish model. Very few to none enjoy being that size, I can think very visibly of the reaction Jae had on her face when she saw she was pushing 700lbs.
I feel like theres a middle ground with women like Boberry who have a very pretty face, take care of themselves and also are massive. Although when Boberry originally dropped from low 600lbs back into the 500s I did stop watching her for awhile and only started paying attention more recently when she began gaining again.
In reality I would never date a women over 400lbs and I wouldn't want my current girlfriend to be fast tracking to 500-600lbs either. It's fun to fantasize about it online and I'd probably fuck a USSBBW but the issues especially the hygiene and health are not something I'd be prepared to deal with in real life.