Did you release this earlier than today? I'm pretty sure that I listened to this particular podcast last week. Anyhow, it was a good episode!
I think fan art is a really fun thing to do as a fan who really appreciates something. It's a way of honoring something you love. I have a lot of opinions about it. This will probably be a long post 
I know of many artists who just do fan art and that's their entire career. Yes, many of them try changing it with a new style or point of view, but they rarely if ever make any personal work. Their following is purely based on their fan art, and therefore all their work is always reactionary to something someone else puts out and can get stale really fast. Your fans are not fans of your work, but of the IP that you are using. That's just a bummer to me.
However, I have a friend who did fanart sculptures and put them in her portfolio. She got approached by someone to make officially licensed work, got an internship working for a company who works on product for that IP, and is trying to get a license to sell certain pieces. But her fanart was very different than what is typically seen and filled a place that was currently empty in the market, and showed skills that those people were looking for. Disney and other companies have licensing divisions that just work on product for their parks and stores or certain brands etc that is basically making fan art for a job. I've heard first hand that they like seeing their licensed characters in portfolios. But that's done for fun and to get a job with those companies, and not to sell as your own.
I'm not a fan, no pun intended, of selling something with someone else's intellectual property. It seems skeevy to me to essentially rip off someone else's idea. It doesn't matter to me whether it's a big corporation or a small indie creator. If you really are a fan of something, isn't it better to support it with your own money so they can continue creating what you love? It seems sort of hypocritical to me that as artists we can talk about copyright law and protecting our work till the cows come home, but when it comes to a big corporation's ideas or an IP we like, it's open season. I want my work to be protected, so I should treat other people's work with that same respect.
I think people should make work based on what inspires them. I don't think fanart is bad at all. I think it's great when you love something a lot to express it. I love fanart actually. I wish there was more of it for things I like.
I wish people would take more creative risks when they do it as an exercise to stretch themselves. It's similar to a master copy to me. Do it, learn, have fun with it, but you'd never sell it as your own. I think it's a great way to expand your brand; a great example of this is Loish (she made a lot of fanart mixed with personal work early on and still does fanart on occasion, but now people follow her for her own content). But I don't think it's right or legal to sell it as your own without permission. Sure there is a lot of gray area, but I wouldn't bother.
I do wish there was a lot more fanart of music and books and poetry and food and things that people love but don't have a visual language assigned to them. Those always seem more fascinating to me.
TL;DR: In my opinion, making fan art is not the problem, and can have benefits. It's a great way to express your love for something. It's a great way to keep inspired and a great way to learn. However, it can be used as a crutch to gain followers, attention, and ideas, and when done too much it isn't the best for building a sustainable flexible innovative career. It is also not the best idea to sell it, for legal reasons and for the reason that it's better to be financially secure in your own projects rather than someone else's IP, unless you are working for them.
But that's just my long verbose opinion 